cachefiles: fix memory leak in cachefiles_add_cache()

The following memory leak was reported after unbinding /dev/cachefiles:

==================================================================
unreferenced object 0xffff9b674176e3c0 (size 192):
comm "cachefilesd2", pid 680, jiffies 4294881224
hex dump (first 32 bytes):
01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
backtrace (crc ea38a44b):
[<ffffffff8eb8a1a5>] kmem_cache_alloc+0x2d5/0x370
[<ffffffff8e917f86>] prepare_creds+0x26/0x2e0
[<ffffffffc002eeef>] cachefiles_determine_cache_security+0x1f/0x120
[<ffffffffc00243ec>] cachefiles_add_cache+0x13c/0x3a0
[<ffffffffc0025216>] cachefiles_daemon_write+0x146/0x1c0
[<ffffffff8ebc4a3b>] vfs_write+0xcb/0x520
[<ffffffff8ebc5069>] ksys_write+0x69/0xf0
[<ffffffff8f6d4662>] do_syscall_64+0x72/0x140
[<ffffffff8f8000aa>] entry_SYSCALL_64_after_hwframe+0x6e/0x76
==================================================================

Put the reference count of cache_cred in cachefiles_daemon_unbind() to
fix the problem. And also put cache_cred in cachefiles_add_cache() error
branch to avoid memory leaks.

Fixes: 9ae326a69004 ("CacheFiles: A cache that backs onto a mounted filesystem")
CC: stable@vger.kernel.org
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Link: https://lore.kernel.org/r/20240217081431.796809-1-libaokun1@huawei.com
Acked-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

cachefiles: Check that the backing filesystem supports tmpfiles

Add a check that the backing filesystem supports the creation of
tmpfiles[1].

Suggested-by: Jeff Layton <jlayton@kernel.org>
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/568749bd7cc02908ecf6f3d6a611b6f9cf5c4afd.camel@kernel.org/ [1]
Link: https://lore.kernel.org/r/164251406558.3435901.1249023136670058162.stgit@warthog.procyon.org.uk/ # v1

cachefiles: Explain checks in a comment

Add a comment to explain the checks that cachefiles is making of the
backing filesystem[1].

Suggested-by: Jeff Layton <jlayton@kernel.org>
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/568749bd7cc02908ecf6f3d6a611b6f9cf5c4afd.camel@kernel.org/ [1]
Link: https://lore.kernel.org/r/164251405621.3435901.771439791811515914.stgit@warthog.procyon.org.uk/ # v1

cachefiles: Calculate the blockshift in terms of bytes, not pages

Cachefiles keeps track of how much space is available on the backing
filesystem and refuses new writes permission to start if there isn't enough
(we especially don't want ENOSPC happening). It also tracks the amount of
data pending in DIO writes (cache->b_writing) and reduces the amount of
free space available by this amount before deciding if it can set up a new
write.

However, the old fscache I/O API was very much page-granularity dependent
and, as such, cachefiles's cache->bshift was meant to be a multiplier to
get from PAGE_SIZE to block size (ie. a blocksize of 512 would give a shift
of 3 for a 4KiB page) - and this was incorrectly being used to turn the
number of bytes in a DIO write into a number of blocks, leading to a
massive over estimation of the amount of data in flight.

Fix this by changing cache->bshift to be a multiplier from bytes to
blocksize and deal with quantities of blocks, not quantities of pages.

Fix also the rounding in the calculation in cachefiles_write() which needs
a "- 1" inserting.

Fixes: 047487c947e8 ("cachefiles: Implement the I/O routines")
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/164251398954.3435901.7138806620218474123.stgit@warthog.procyon.org.uk/ # v1

fscache, cachefiles: Display stats of no-space events

Add stat counters of no-space events that caused caching not to happen and
display in /proc/fs/fscache/stats.

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/163819653216.215744.17210522251617386509.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906958369.143852.7257100711818401748.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967166917.1823006.14842444049198947892.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021566184.640689.4417328329632709265.stgit@warthog.procyon.org.uk/ # v4

cachefiles: Implement backing file wrangling

Implement the wrangling of backing files, including the following pieces:

(1) Lookup and creation of a file on disk, using a tmpfile if the file
isn't yet present. The file is then opened, sized for DIO and the
file handle is attached to the cachefiles_object struct. The inode is
marked to indicate that it's in use by a kernel service.

(2) Invalidation of an object, creating a tmpfile and switching the file
pointer in the cachefiles object.

(3) Committing a file to disk, including setting the coherency xattr on it
and, if necessary, creating a hard link to it.

Note that this would be a good place to use Omar Sandoval's vfs_link()
with AT_LINK_REPLACE[1] as I may have to unlink an old file before I
can link a tmpfile into place.

(4) Withdrawal of open objects when a cache is being withdrawn or a cookie
is relinquished. This involves committing or discarding the file.

Changes
=======
ver #2:
- Fix logging of wrong error[1].

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/20211203094950.GA2480@kili/ [1]
Link: https://lore.kernel.org/r/163819644097.215744.4505389616742411239.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906949512.143852.14222856795032602080.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967158526.1823006.17482695321424642675.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021557060.640689.16373541458119269871.stgit@warthog.procyon.org.uk/ # v4

cachefiles: Implement volume support

Implement support for creating the directory layout for a volume on disk
and setting up and withdrawing volume caching.

Each volume has a directory named for the volume key under the root of the
cache (prefixed with an 'I' to indicate to cachefilesd that it's an index)
and then creates a bunch of hash bucket subdirectories under that (named as
'@' plus a hex number) in which cookie files will be created.

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/163819635314.215744.13081522301564537723.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906936397.143852.17788457778396467161.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967143860.1823006.7185205806080225038.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021545212.640689.5064821392307582927.stgit@warthog.procyon.org.uk/ # v4

cachefiles: Implement cache registration and withdrawal

Do the following:

(1) Fill out cachefiles_daemon_add_cache() so that it sets up the cache
directories and registers the cache with cachefiles.

(2) Add a function to do the top-level part of cache withdrawal and
unregistration.

(3) Add a function to sync a cache.

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/163819633175.215744.10857127598041268340.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906935445.143852.15545194974036410029.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967142904.1823006.244055483596047072.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021543872.640689.14370017789605073222.stgit@warthog.procyon.org.uk/ # v4

cachefiles: Provide a function to check how much space there is

Provide a function to check how much space there is. This also flips the
state on the cache and will signal the daemon to inform it of the change
and to ask it to do some culling if necessary.

We will also need to subtract the amount of data currently being written to
the cache (cache->b_writing) from the amount of available space to avoid
hitting ENOSPC accidentally.

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/163819629322.215744.13457425294680841213.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906930100.143852.1681026700865762069.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967140058.1823006.7781243664702837128.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021539957.640689.12477177372616805706.stgit@warthog.procyon.org.uk/ # v4