On Tue, Apr 07, 2020 at 09:30:02AM +1000, Dave Chinner wrote: > On Mon, Apr 06, 2020 at 10:06:03AM -0700, Darrick J. Wong wrote: > > On Sat, Apr 04, 2020 at 02:22:03PM +0530, Chandan Rajendra wrote: > > > XFS has a per-inode xattr extent counter which is 16 bits wide. A workload > > > which > > > 1. Creates 1,000,000 255-byte sized xattrs, > > > 2. Deletes 50% of these xattrs in an alternating manner, > > > 3. Tries to create 400,000 new 255-byte sized xattrs > > > causes the following message to be printed on the console, > > > > > > XFS (loop0): xfs_iflush_int: detected corrupt incore inode 131, total extents = -19916, nblocks = 102937, ptr ffff9ce33b098c00 > > > XFS (loop0): xfs_do_force_shutdown(0x8) called from line 3739 of file fs/xfs/xfs_inode.c. Return address = ffffffffa4a94173 > > > > > > This indicates that we overflowed the 16-bits wide xattr extent counter. > > > > > > I have been informed that there are instances where a single file has > > > > 100 million hardlinks. With parent pointers being stored in xattr, > > > we will overflow the 16-bits wide xattr extent counter when large > > > number of hardlinks are created. > > > > > > Hence this commit extends xattr extent counter to 32-bits. It also introduces > > > an incompat flag to prevent older kernels from mounting newer filesystems with > > > 32-bit wide xattr extent counter. > > > > > > Signed-off-by: Chandan Rajendra <chandanrlinux@xxxxxxxxx> > > > --- > > > fs/xfs/libxfs/xfs_format.h | 28 +++++++++++++++++++++------- > > > fs/xfs/libxfs/xfs_inode_buf.c | 27 +++++++++++++++++++-------- > > > fs/xfs/libxfs/xfs_inode_fork.c | 3 ++- > > > fs/xfs/libxfs/xfs_log_format.h | 5 +++-- > > > fs/xfs/libxfs/xfs_types.h | 4 ++-- > > > fs/xfs/scrub/inode.c | 7 ++++--- > > > fs/xfs/xfs_inode_item.c | 3 ++- > > > fs/xfs/xfs_log_recover.c | 13 ++++++++++--- > > > 8 files changed, 63 insertions(+), 27 deletions(-) > > > > > > diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h > > > index 045556e78ee2c..0a4266b0d46e1 100644 > > > --- a/fs/xfs/libxfs/xfs_format.h > > > +++ b/fs/xfs/libxfs/xfs_format.h > > > @@ -465,10 +465,12 @@ xfs_sb_has_ro_compat_feature( > > > #define XFS_SB_FEAT_INCOMPAT_FTYPE (1 << 0) /* filetype in dirent */ > > > #define XFS_SB_FEAT_INCOMPAT_SPINODES (1 << 1) /* sparse inode chunks */ > > > #define XFS_SB_FEAT_INCOMPAT_META_UUID (1 << 2) /* metadata UUID */ > > > +#define XFS_SB_FEAT_INCOMPAT_32BIT_AEXT_CNTR (1 << 3) > > > > If you're going to introduce an INCOMPAT feature, please also use the > > opportunity to convert xattrs to something resembling the dir v3 format, > > where we index free space within each block so that we can speed up attr > > setting with 100 million attrs. > > Not necessary. Chandan has already spent a lot of time investigating > that - I suggested doing the investigation probably a year ago when > he was looking for stuff to do knowing that this could be a problem > parent pointers hit. Oh, I didn't realize that analysis work has already been done. Chandan, could you please mention that somewhere in the cover letter? It does mention that you tried creating 1M xattrs, but I guess it needed to be more explicit about not uncovering any gigantic performance holes. > Long story short - there's no degradation in > performance in the dabtree out to tens of millions of records with > different fixed size or random sized attributes, nor does various > combinations of insert/lookup/remove/replace operations seem to > impact the tree performance at scale. IOWs, we hit the 16 bit extent > limits of the attribute trees without finding any degradation in > performance. Ok. I'll take "attr v3 upgrade" off my list of things to look out for. > Hence we concluded that the dabtree structure does not require > significant modification or optimisation to work well with typical > parent pointer attribute demands... > > As for free space indexes.... > > The issue with the directory structure that requires external free > space is that the directory data is not part of the dabtree itself. > The attribute fork stores all the attributes at the leaves of the > dabtree, while the directory structure stores the directory data in > external blocks and the dabtree only contains the name hash index > that points to the external data. > > i.e. When we add an attribute to the dabtree, we split/merge leaves > of the tree based on where the name hash index tells us it needs to > be inserted/removed from. i.e. we make space available or collapse > sparse leaves of the dabtree as a side effect of inserting or > removing objects. > > The directory structure is very different. The dirents cannot change > location as their logical offset into the dir data segment is used > as the readdir/seekdir/telldir cookie. Therefore that location is > not allowed to change for the life of the dirent and so we can't > store them in the leaves of a dabtree indexed in hash order because > the offset into the tree would change as other entries are inserted > and removed. Hence when we remove dirents, we must leave holes in > the data segment so the rest of the dirent data does not change > logical offset. > > The directory name hash index - the dabtree bit - is in a separate > segment (the 2nd one). Because it only stores pointers to dirents in > the data segment, it doesn't need to leave holes - the dabtree just > merge/splits as required as pointers to the dir data segment are > added/removed - and has no free space tracking. > > Hence when we go to add a dirent, we need to find the best free > space in the dir data segment to add that dirent. This requires a > dir data segment free space index, and that is held in the 3rd dir > segment. Once we've found the best free space via lookup in the > free space index, we go modify the dir data block it points to, then > update the dabtree to point the name hash at that new dirent. > > IOWs, the requirement for a free space map in the directory > structure results from storing the dirent data externally to the > dabtree. Attributes are stored directly in the leaves of the > dabtree - except for remote attributes which can be anywhere in the > BMBT address space - and hence do no need external free space > tracking to determine where to best insert them... <nod> Got it. I've suspected this property about the xattr structures for a long time, so I'm glad to hear someone else echo that. :) Dave: May I try to rework the above into something suitable for the ondisk format documentation? --D > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx
