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H A D | fscache.h | diff b5b52de3 Wed Dec 07 06:49:15 MST 2022 Dave Wysochanski <dwysocha@redhat.com> fscache: Fix oops due to race with cookie_lru and use_cookie If a cookie expires from the LRU and the LRU_DISCARD flag is set, but the state machine has not run yet, it's possible another thread can call fscache_use_cookie and begin to use it. When the cookie_worker finally runs, it will see the LRU_DISCARD flag set, transition the cookie->state to LRU_DISCARDING, which will then withdraw the cookie. Once the cookie is withdrawn the object is removed the below oops will occur because the object associated with the cookie is now NULL. Fix the oops by clearing the LRU_DISCARD bit if another thread uses the cookie before the cookie_worker runs. BUG: kernel NULL pointer dereference, address: 0000000000000008 ... CPU: 31 PID: 44773 Comm: kworker/u130:1 Tainted: G E 6.0.0-5.dneg.x86_64 #1 Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 08/26/2022 Workqueue: events_unbound netfs_rreq_write_to_cache_work [netfs] RIP: 0010:cachefiles_prepare_write+0x28/0x90 [cachefiles] ... Call Trace: netfs_rreq_write_to_cache_work+0x11c/0x320 [netfs] process_one_work+0x217/0x3e0 worker_thread+0x4a/0x3b0 kthread+0xd6/0x100 Fixes: 12bb21a29c19 ("fscache: Implement cookie user counting and resource pinning") Reported-by: Daire Byrne <daire.byrne@gmail.com> Signed-off-by: Dave Wysochanski <dwysocha@redhat.com> Signed-off-by: David Howells <dhowells@redhat.com> Tested-by: Daire Byrne <daire@dneg.com> Link: https://lore.kernel.org/r/20221117115023.1350181-1-dwysocha@redhat.com/ # v1 Link: https://lore.kernel.org/r/20221117142915.1366990-1-dwysocha@redhat.com/ # v2 Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> diff 12bb21a2 Wed Oct 20 08:53:34 MDT 2021 David Howells <dhowells@redhat.com> fscache: Implement cookie user counting and resource pinning Provide a pair of functions to count the number of users of a cookie (open files, writeback, invalidation, resizing, reads, writes), to obtain and pin resources for the cookie and to prevent culling for the whilst there are users. The first function marks a cookie as being in use: void fscache_use_cookie(struct fscache_cookie *cookie, bool will_modify); The caller should indicate the cookie to use and whether or not the caller is in a context that may modify the cookie (e.g. a file open O_RDWR). If the cookie is not already resourced, fscache will ask the cache backend in the background to do whatever it needs to look up, create or otherwise obtain the resources necessary to access data. This is pinned to the cookie and may not be culled, though it may be withdrawn if the cache as a whole is withdrawn. The second function removes the in-use mark from a cookie and, optionally, updates the coherency data: void fscache_unuse_cookie(struct fscache_cookie *cookie, const void *aux_data, const loff_t *object_size); If non-NULL, the aux_data buffer and/or the object_size will be saved into the cookie and will be set on the backing store when the object is committed. If this removes the last usage on a cookie, the cookie is placed onto an LRU list from which it will be removed and closed after a couple of seconds if it doesn't get reused. This prevents resource overload in the cache - in particular it prevents it from holding too many files open. Changes ======= ver #2: - Fix fscache_unuse_cookie() to use atomic_dec_and_lock() to avoid a potential race if the cookie gets reused before it completes the unusement. - Added missing transition to LRU_DISCARDING state. Signed-off-by: David Howells <dhowells@redhat.com> Reviewed-by: Jeff Layton <jlayton@kernel.org> cc: linux-cachefs@redhat.com Link: https://lore.kernel.org/r/163819600612.215744.13678350304176542741.stgit@warthog.procyon.org.uk/ # v1 Link: https://lore.kernel.org/r/163906907567.143852.16979631199380722019.stgit@warthog.procyon.org.uk/ # v2 Link: https://lore.kernel.org/r/163967106467.1823006.6790864931048582667.stgit@warthog.procyon.org.uk/ # v3 Link: https://lore.kernel.org/r/164021511674.640689.10084988363699111860.stgit@warthog.procyon.org.uk/ # v4 diff 12bb21a2 Wed Oct 20 08:53:34 MDT 2021 David Howells <dhowells@redhat.com> fscache: Implement cookie user counting and resource pinning Provide a pair of functions to count the number of users of a cookie (open files, writeback, invalidation, resizing, reads, writes), to obtain and pin resources for the cookie and to prevent culling for the whilst there are users. The first function marks a cookie as being in use: void fscache_use_cookie(struct fscache_cookie *cookie, bool will_modify); The caller should indicate the cookie to use and whether or not the caller is in a context that may modify the cookie (e.g. a file open O_RDWR). If the cookie is not already resourced, fscache will ask the cache backend in the background to do whatever it needs to look up, create or otherwise obtain the resources necessary to access data. This is pinned to the cookie and may not be culled, though it may be withdrawn if the cache as a whole is withdrawn. The second function removes the in-use mark from a cookie and, optionally, updates the coherency data: void fscache_unuse_cookie(struct fscache_cookie *cookie, const void *aux_data, const loff_t *object_size); If non-NULL, the aux_data buffer and/or the object_size will be saved into the cookie and will be set on the backing store when the object is committed. If this removes the last usage on a cookie, the cookie is placed onto an LRU list from which it will be removed and closed after a couple of seconds if it doesn't get reused. This prevents resource overload in the cache - in particular it prevents it from holding too many files open. Changes ======= ver #2: - Fix fscache_unuse_cookie() to use atomic_dec_and_lock() to avoid a potential race if the cookie gets reused before it completes the unusement. - Added missing transition to LRU_DISCARDING state. Signed-off-by: David Howells <dhowells@redhat.com> Reviewed-by: Jeff Layton <jlayton@kernel.org> cc: linux-cachefs@redhat.com Link: https://lore.kernel.org/r/163819600612.215744.13678350304176542741.stgit@warthog.procyon.org.uk/ # v1 Link: https://lore.kernel.org/r/163906907567.143852.16979631199380722019.stgit@warthog.procyon.org.uk/ # v2 Link: https://lore.kernel.org/r/163967106467.1823006.6790864931048582667.stgit@warthog.procyon.org.uk/ # v3 Link: https://lore.kernel.org/r/164021511674.640689.10084988363699111860.stgit@warthog.procyon.org.uk/ # v4 |
/linux-master/include/linux/ | ||
H A D | fscache.h | diff 12bb21a2 Wed Oct 20 08:53:34 MDT 2021 David Howells <dhowells@redhat.com> fscache: Implement cookie user counting and resource pinning Provide a pair of functions to count the number of users of a cookie (open files, writeback, invalidation, resizing, reads, writes), to obtain and pin resources for the cookie and to prevent culling for the whilst there are users. The first function marks a cookie as being in use: void fscache_use_cookie(struct fscache_cookie *cookie, bool will_modify); The caller should indicate the cookie to use and whether or not the caller is in a context that may modify the cookie (e.g. a file open O_RDWR). If the cookie is not already resourced, fscache will ask the cache backend in the background to do whatever it needs to look up, create or otherwise obtain the resources necessary to access data. This is pinned to the cookie and may not be culled, though it may be withdrawn if the cache as a whole is withdrawn. The second function removes the in-use mark from a cookie and, optionally, updates the coherency data: void fscache_unuse_cookie(struct fscache_cookie *cookie, const void *aux_data, const loff_t *object_size); If non-NULL, the aux_data buffer and/or the object_size will be saved into the cookie and will be set on the backing store when the object is committed. If this removes the last usage on a cookie, the cookie is placed onto an LRU list from which it will be removed and closed after a couple of seconds if it doesn't get reused. This prevents resource overload in the cache - in particular it prevents it from holding too many files open. Changes ======= ver #2: - Fix fscache_unuse_cookie() to use atomic_dec_and_lock() to avoid a potential race if the cookie gets reused before it completes the unusement. - Added missing transition to LRU_DISCARDING state. Signed-off-by: David Howells <dhowells@redhat.com> Reviewed-by: Jeff Layton <jlayton@kernel.org> cc: linux-cachefs@redhat.com Link: https://lore.kernel.org/r/163819600612.215744.13678350304176542741.stgit@warthog.procyon.org.uk/ # v1 Link: https://lore.kernel.org/r/163906907567.143852.16979631199380722019.stgit@warthog.procyon.org.uk/ # v2 Link: https://lore.kernel.org/r/163967106467.1823006.6790864931048582667.stgit@warthog.procyon.org.uk/ # v3 Link: https://lore.kernel.org/r/164021511674.640689.10084988363699111860.stgit@warthog.procyon.org.uk/ # v4 diff 12bb21a2 Wed Oct 20 08:53:34 MDT 2021 David Howells <dhowells@redhat.com> fscache: Implement cookie user counting and resource pinning Provide a pair of functions to count the number of users of a cookie (open files, writeback, invalidation, resizing, reads, writes), to obtain and pin resources for the cookie and to prevent culling for the whilst there are users. The first function marks a cookie as being in use: void fscache_use_cookie(struct fscache_cookie *cookie, bool will_modify); The caller should indicate the cookie to use and whether or not the caller is in a context that may modify the cookie (e.g. a file open O_RDWR). If the cookie is not already resourced, fscache will ask the cache backend in the background to do whatever it needs to look up, create or otherwise obtain the resources necessary to access data. This is pinned to the cookie and may not be culled, though it may be withdrawn if the cache as a whole is withdrawn. The second function removes the in-use mark from a cookie and, optionally, updates the coherency data: void fscache_unuse_cookie(struct fscache_cookie *cookie, const void *aux_data, const loff_t *object_size); If non-NULL, the aux_data buffer and/or the object_size will be saved into the cookie and will be set on the backing store when the object is committed. If this removes the last usage on a cookie, the cookie is placed onto an LRU list from which it will be removed and closed after a couple of seconds if it doesn't get reused. This prevents resource overload in the cache - in particular it prevents it from holding too many files open. Changes ======= ver #2: - Fix fscache_unuse_cookie() to use atomic_dec_and_lock() to avoid a potential race if the cookie gets reused before it completes the unusement. - Added missing transition to LRU_DISCARDING state. Signed-off-by: David Howells <dhowells@redhat.com> Reviewed-by: Jeff Layton <jlayton@kernel.org> cc: linux-cachefs@redhat.com Link: https://lore.kernel.org/r/163819600612.215744.13678350304176542741.stgit@warthog.procyon.org.uk/ # v1 Link: https://lore.kernel.org/r/163906907567.143852.16979631199380722019.stgit@warthog.procyon.org.uk/ # v2 Link: https://lore.kernel.org/r/163967106467.1823006.6790864931048582667.stgit@warthog.procyon.org.uk/ # v3 Link: https://lore.kernel.org/r/164021511674.640689.10084988363699111860.stgit@warthog.procyon.org.uk/ # v4 2d6fff63 Fri Apr 03 09:42:36 MDT 2009 David Howells <dhowells@redhat.com> FS-Cache: Add the FS-Cache netfs API and documentation Add the API for a generic facility (FS-Cache) by which filesystems (such as AFS or NFS) may call on local caching capabilities without having to know anything about how the cache works, or even if there is a cache: +---------+ | | +--------------+ | NFS |--+ | | | | | +-->| CacheFS | +---------+ | +----------+ | | /dev/hda5 | | | | | +--------------+ +---------+ +-->| | | | | | |--+ | AFS |----->| FS-Cache | | | | |--+ +---------+ +-->| | | | | | | +--------------+ +---------+ | +----------+ | | | | | | +-->| CacheFiles | | ISOFS |--+ | /var/cache | | | +--------------+ +---------+ General documentation and documentation of the netfs specific API are provided in addition to the header files. As this patch stands, it is possible to build a filesystem against the facility and attempt to use it. All that will happen is that all requests will be immediately denied as if no cache is present. Further patches will implement the core of the facility. The facility will transfer requests from networking filesystems to appropriate caches if possible, or else gracefully deny them. If this facility is disabled in the kernel configuration, then all its operations will trivially reduce to nothing during compilation. WHY NOT I_MAPPING? ================== I have added my own API to implement caching rather than using i_mapping to do this for a number of reasons. These have been discussed a lot on the LKML and CacheFS mailing lists, but to summarise the basics: (1) Most filesystems don't do hole reportage. Holes in files are treated as blocks of zeros and can't be distinguished otherwise, making it difficult to distinguish blocks that have been read from the network and cached from those that haven't. (2) The backing inode must be fully populated before being exposed to userspace through the main inode because the VM/VFS goes directly to the backing inode and does not interrogate the front inode's VM ops. Therefore: (a) The backing inode must fit entirely within the cache. (b) All backed files currently open must fit entirely within the cache at the same time. (c) A working set of files in total larger than the cache may not be cached. (d) A file may not grow larger than the available space in the cache. (e) A file that's open and cached, and remotely grows larger than the cache is potentially stuffed. (3) Writes go to the backing filesystem, and can only be transferred to the network when the file is closed. (4) There's no record of what changes have been made, so the whole file must be written back. (5) The pages belong to the backing filesystem, and all metadata associated with that page are relevant only to the backing filesystem, and not anything stacked atop it. OVERVIEW ======== FS-Cache provides (or will provide) the following facilities: (1) Caches can be added / removed at any time, even whilst in use. (2) Adds a facility by which tags can be used to refer to caches, even if they're not available yet. (3) More than one cache can be used at once. Caches can be selected explicitly by use of tags. (4) The netfs is provided with an interface that allows either party to withdraw caching facilities from a file (required for (1)). (5) A netfs may annotate cache objects that belongs to it. This permits the storage of coherency maintenance data. (6) Cache objects will be pinnable and space reservations will be possible. (7) The interface to the netfs returns as few errors as possible, preferring rather to let the netfs remain oblivious. (8) Cookies are used to represent indices, files and other objects to the netfs. The simplest cookie is just a NULL pointer - indicating nothing cached there. (9) The netfs is allowed to propose - dynamically - any index hierarchy it desires, though it must be aware that the index search function is recursive, stack space is limited, and indices can only be children of indices. (10) Indices can be used to group files together to reduce key size and to make group invalidation easier. The use of indices may make lookup quicker, but that's cache dependent. (11) Data I/O is effectively done directly to and from the netfs's pages. The netfs indicates that page A is at index B of the data-file represented by cookie C, and that it should be read or written. The cache backend may or may not start I/O on that page, but if it does, a netfs callback will be invoked to indicate completion. The I/O may be either synchronous or asynchronous. (12) Cookies can be "retired" upon release. At this point FS-Cache will mark them as obsolete and the index hierarchy rooted at that point will get recycled. (13) The netfs provides a "match" function for index searches. In addition to saying whether a match was made or not, this can also specify that an entry should be updated or deleted. FS-Cache maintains a virtual index tree in which all indices, files, objects and pages are kept. Bits of this tree may actually reside in one or more caches. FSDEF | +------------------------------------+ | | NFS AFS | | +--------------------------+ +-----------+ | | | | homedir mirror afs.org redhat.com | | | +------------+ +---------------+ +----------+ | | | | | | 00001 00002 00007 00125 vol00001 vol00002 | | | | | +---+---+ +-----+ +---+ +------+------+ +-----+----+ | | | | | | | | | | | | | PG0 PG1 PG2 PG0 XATTR PG0 PG1 DIRENT DIRENT DIRENT R/W R/O Bak | | PG0 +-------+ | | 00001 00003 | +---+---+ | | | PG0 PG1 PG2 In the example above, two netfs's can be seen to be backed: NFS and AFS. These have different index hierarchies: (*) The NFS primary index will probably contain per-server indices. Each server index is indexed by NFS file handles to get data file objects. Each data file objects can have an array of pages, but may also have further child objects, such as extended attributes and directory entries. Extended attribute objects themselves have page-array contents. (*) The AFS primary index contains per-cell indices. Each cell index contains per-logical-volume indices. Each of volume index contains up to three indices for the read-write, read-only and backup mirrors of those volumes. Each of these contains vnode data file objects, each of which contains an array of pages. The very top index is the FS-Cache master index in which individual netfs's have entries. Any index object may reside in more than one cache, provided it only has index children. Any index with non-index object children will be assumed to only reside in one cache. The FS-Cache overview can be found in: Documentation/filesystems/caching/fscache.txt The netfs API to FS-Cache can be found in: Documentation/filesystems/caching/netfs-api.txt Signed-off-by: David Howells <dhowells@redhat.com> Acked-by: Steve Dickson <steved@redhat.com> Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com> Acked-by: Al Viro <viro@zeniv.linux.org.uk> Tested-by: Daire Byrne <Daire.Byrne@framestore.com> |
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