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authorVivek Goyal <vgoyal@redhat.com>2020-10-09 21:15:12 +0300
committerMiklos Szeredi <mszeredi@redhat.com>2020-11-11 19:22:33 +0300
commit9d769e6aa2524e1762e3b8681e0ed78f8acf6cad (patch)
tree74e8d77ec19602d74646e46a74be1342495e3dd4 /fs/fuse/readdir.c
parent643a666a89c358ef588d2b3ef9f2dc1efc421e61 (diff)
downloadlinux-9d769e6aa2524e1762e3b8681e0ed78f8acf6cad.tar.xz
fuse: support SB_NOSEC flag to improve write performance
Virtiofs can be slow with small writes if xattr are enabled and we are doing cached writes (No direct I/O). Ganesh Mahalingam noticed this. Some debugging showed that file_remove_privs() is called in cached write path on every write. And everytime it calls security_inode_need_killpriv() which results in call to __vfs_getxattr(XATTR_NAME_CAPS). And this goes to file server to fetch xattr. This extra round trip for every write slows down writes tremendously. Normally to avoid paying this penalty on every write, vfs has the notion of caching this information in inode (S_NOSEC). So vfs sets S_NOSEC, if filesystem opted for it using super block flag SB_NOSEC. And S_NOSEC is cleared when setuid/setgid bit is set or when security xattr is set on inode so that next time a write happens, we check inode again for clearing setuid/setgid bits as well clear any security.capability xattr. This seems to work well for local file systems but for remote file systems it is possible that VFS does not have full picture and a different client sets setuid/setgid bit or security.capability xattr on file and that means VFS information about S_NOSEC on another client will be stale. So for remote filesystems SB_NOSEC was disabled by default. Commit 9e1f1de02c22 ("more conservative S_NOSEC handling") mentioned that these filesystems can still make use of SB_NOSEC as long as they clear S_NOSEC when they are refreshing inode attriutes from server. So this patch tries to enable SB_NOSEC on fuse (regular fuse as well as virtiofs). And clear SB_NOSEC when we are refreshing inode attributes. This is enabled only if server supports FUSE_HANDLE_KILLPRIV_V2. This says that server will clear setuid/setgid/security.capability on chown/truncate/write as apporpriate. This should provide tighter coherency because now suid/sgid/ security.capability will be cleared even if fuse client cache has not seen these attrs. Basic idea is that fuse client will trigger suid/sgid/security.capability clearing based on its attr cache. But even if cache has gone stale, it is fine because FUSE_HANDLE_KILLPRIV_V2 will make sure WRITE clear suid/sgid/security.capability. We make this change only if server supports FUSE_HANDLE_KILLPRIV_V2. This should make sure that existing filesystems which might be relying on seucurity.capability always being queried from server are not impacted. This tighter coherency relies on WRITE showing up on server (and not being cached in guest). So writeback_cache mode will not provide that tight coherency and it is not recommended to use two together. Having said that it might work reasonably well for lot of use cases. This change improves random write performance very significantly. Running virtiofsd with cache=auto and following fio command: fio --ioengine=libaio --direct=1 --name=test --filename=/mnt/virtiofs/random_read_write.fio --bs=4k --iodepth=64 --size=4G --readwrite=randwrite Bandwidth increases from around 50MB/s to around 250MB/s as a result of applying this patch. So improvement is very significant. Link: https://github.com/kata-containers/runtime/issues/2815 Reported-by: "Mahalingam, Ganesh" <ganesh.mahalingam@intel.com> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Diffstat (limited to 'fs/fuse/readdir.c')
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