On Tue, Nov 28, 2017 at 08:57:48AM -0500, Brian Foster wrote: > On Tue, Nov 28, 2017 at 10:07:34AM +1100, Dave Chinner wrote: > > On Mon, Nov 27, 2017 at 03:24:33PM -0500, Brian Foster wrote: > > > We've had rare reports of transaction overruns in > > > xfs_inactive_ifree() for quite some time. Analysis of a reproducing > > > metadump has shown the problem is essentially caused by performing > > > too many agfl block frees in a single transaction. > > > > > > For example, an inode chunk is freed and the associated agfl fixup > > > algorithm discovers it needs to free a single agfl block before the > > > chunk free occurs. This single block ends up causing a space btree > > > join and adds one or more blocks back onto the agfl. This causes > > > xfs_alloc_fix_freelist() to free up to 3 blocks just to rectify a > > > single block discrepency. > > > > > > The transaction overrun occurs under several other unfortunate > > > circumstances: > > > > > > - Each agfl block free is left+right contiguous. This requires 2 > > > record deletions and 1 insertion for the cntbt and thus requires > > > more log reservation than normal. > > > - The associated transaction is the first in the CIL ctx and thus > > > the ctx header reservation is consumed. > > > - The transaction reservation is larger than a log buffer and thus > > > extra split header reservation is consumed. > > > > > > As a result of the agfl and free space state of the filesystem, the > > > agfl fixup code has dirtied more cntbt buffer space than allowed by > > > the portion of the reservation allotted for block allocation. This > > > is all before the real allocation even starts! > > > > > > Note that the log related conditions above are correctly covered by > > > the existing transaction reservation. The above demonstrates that > > > the reservation allotted for the context/split headers may help > > > suppress overruns in the more common case where that reservation > > > goes unused for its intended purpose. > > > > > > To address this problem, update xfs_alloc_fix_freelist() to amortize > > > agfl block frees over multiple transactions. Free one block per > > > transaction so long as the agfl is less than half free. The agfl > > > minimum allocation requirement is dynamic, but is based on the > > > geometry of the associated btrees (i.e., level count) and therefore > > > should be easily rectified over multiple allocation transactions. > > > Further, there is no real harm in leaving extraneous blocks on the > > > agfl so long as there are enough free slots available for btree > > > blocks freed as a result of the upcoming allocation. > > > > > > Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx> > > > --- > > > fs/xfs/libxfs/xfs_alloc.c | 15 ++++++++++----- > > > 1 file changed, 10 insertions(+), 5 deletions(-) > > > > > > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c > > > index 0da80019a917..d8d58e35da00 100644 > > > --- a/fs/xfs/libxfs/xfs_alloc.c > > > +++ b/fs/xfs/libxfs/xfs_alloc.c > > > @@ -2117,11 +2117,6 @@ xfs_alloc_fix_freelist( > > > * appropriately based on the recursion count and dirty state of the > > > * buffer. > > > * > > > - * XXX (dgc): When we have lots of free space, does this buy us > > > - * anything other than extra overhead when we need to put more blocks > > > - * back on the free list? Maybe we should only do this when space is > > > - * getting low or the AGFL is more than half full? > > > - * > > > * The NOSHRINK flag prevents the AGFL from being shrunk if it's too > > > * big; the NORMAP flag prevents AGFL expand/shrink operations from > > > * updating the rmapbt. Both flags are used in xfs_repair while we're > > > @@ -2151,6 +2146,16 @@ xfs_alloc_fix_freelist( > > > goto out_agbp_relse; > > > } > > > xfs_trans_binval(tp, bp); > > > + > > > + /* > > > + * Freeing all extra agfl blocks adds too much log reservation > > > + * overhead to a single transaction, particularly considering > > > + * that freeing a block can cause a btree join and put one right > > > + * back on the agfl. Try to free one block per tx so long as > > > + * we've left enough free slots for the upcoming modifications. > > > + */ > > > + if (pag->pagf_flcount <= (XFS_AGFL_SIZE(mp) >> 1)) > > > + break; > > > } > > > > In /theory/, this /should/ work. However, as the comment you removed > > implies, there are likely to be issues with this as we get near > > ENOSPC. We know that if we don't trim the AGFL right down to the > > minimum requested as we approach ENOSPC we can get premature ENOSPC > > events being reported that lead to filesystem shutdowns. (e.g. need > > the very last free block for a BMBT block to complete a data extent > > allocation). Hence I'd suggest that this needs to be aware of the > > low space allocation algorithm (i.e. dfops->dop_low is true) to trim > > the agfl right back when we are really short of space > > > > Hmm, shouldn't the worst case bmbt requirements be satisfied by the > block reservation of the transaction that maps the extent (or stored as > indirect reservation if delalloc)? That's checked against global free space when we do the initial reservation. It's not checked against AG free space until we make the allocation attempt. The problem here is that if we don't leave enough space for the BMBT block in the same AG as the data extent was allocated we can end up with an ENOSPC on the BMBT block allocation because, say, AGF locking order and all the higher AGFs are already ENOSPC. That's a fatal ENOSPC error as the transaction is already dirty and will shutdown the filesystem on transaction cancel. > I'm less concerned with premature > ENOSPC from contexts where it's not fatal.. The definition of "premature ENOSPC" is "unexpected ENOSPC that causes a filesystem shutdown" :/ > > IIRC, I think we've already > incorporated changes (Christoph's minfree stuff rings a bell) into the > allocator that explicitly prefer premature ENOSPC over potentially fatal > conditions, but I could be mistaken. Yes, but that doesn't take into account all the crazy AGFL manipulations that could occur as a result of trimming/extending the AGFL. We /assume/ that we don't need signification reservations to extend/trim the AGFL to what is required for the transaction. If we now how to free 50 blocks from the AGFL back to the freespace tree before we start the allocation/free operation, that's going to have some impact on the reservations we have. And think about 4k sector filesystems - there's close on 1000 entries in the AGFL - this means we could have 500 blocks floating on the AGFL that would otherwise be in the freespace tree. IOWs, this change could have significant impact on freespace fragmentation because blocks aren't getting returned to the freespace tree where they merge with other small freespace extents to reform larger extents. It's these sorts of issues (i.e. filesystem aging) that might not show up for months of production use that we need to think about. It may be that a larger AGFL helps here because it keeps a working set of freespace tree blocks on the AGFL rather than having to go back to the freespace trees all the time, but this is something we need to measure, analyse and characterise before changing a critical piece of the allocation architecture.... > We're also only talking about 256k > or so for half of the AGFL, less than that if we assume that some of > those blocks are actually required by the agfl and not slated to be > lazily freed. We could also think about explicitly fixing up agfl > surplus from other contexts (background, write -ENOSPC handling, etc.) > if it became that much of a problem. > > I do think it's semi-reasonable from a conservative standpoint to > further restrict this behavior to !dop_low conditions, but I'm a little > concerned about creating an "open the floodgates" type situation when > the agfl is allowed to carry extra blocks and then all of a sudden the > free space state changes and one transaction is allowed to free a bunch > of blocks. Thoughts? That's exactly the sort of problem we need to measure, analyse and characterise before making this sort of change :/ > I suppose we could incorporate logic that frees until/unless a join > occurs (i.e., the last free did not drop flcount), the latter being an > indication that we've probably logged as much as we should for agfl > fixups in the transaction. But that's also something we could just do > unconditionally as opposed to only under dop_low conditions. That might > be less aggressive of a change from current behavior. Heuristics will still need to be analysed and tested :/ The complexity of doing this is why I left a small comment rather than actually making the change.... > > I'm also concerned that it doesn't take into account that freeing > > a block from the AGFL could cause a freespace tree split to occur, > > thereby emptying the AGFL whilst consuming the entire log > > reservation for tree modifications. This leaves nothing in the log > > reservation for re-growing the AGFL to the minimum required, which > > we /must do/ before returning and could cause more splits/joins to > > occur. > > > > How is this different from the current code? It's not. I'm pointing out that while you're focussed on the problems with shortening the AGFL, the same problem exists with /extending the AGFL/. > This sounds to me like an > unconditional side effect of the fact that freeing an agfl block can > indirectly affect the agfl via the btree operations. IOW, freeing a > single extra block could require consuming one or more and trigger the > need for an allocation. I suspect the allocation could then very well > cause a join on the other tree and put more than one block back onto the > agfl. Yes, it could do that too. Remove a single block from an existing free extent, no change to the by-block btree. by-cnt now requires a record delete (full height join) followed by record insert elsewhere in the tree (full height split). So the attempt to add a block to the AGFL can actually shorten it if the by-cnt tree splits on insert. It can grow if the by-block or by-cnt tree joins on record removal. Either way, we've got the same problem of using the entire log reservation for AGFL modification when growing the AGFL as we do when trying to shrink the AGFL down. That's my point here - just hacking a limit into the shrink case doesn't address the problem at all - it just papers over one of the visible symptoms.... > > IMO, there's a lot more to be concerned about here than just trying > > to work around the specific symptom observed in the given test case. > > This code is, unfortunately, really tricky and intricate and history > > tells us that the risk of unexpected regressions is extremely high, > > especially around ENOSPC related issues. Of all the patches in this > > series, this is the most dangerous and "iffy" of them and the > > one we should be most concerned and conservative about.... > > > > Agreed. The impact is something that also has to be evaluated over a > sequence of transactions along with the more obvious impact on a single > transaction. The impact has to be measured over a far longer time frame than a few transactions. The fact it has impact on freespace reformation means it'll need accelerated aging tests done on it so we can be reasonably certain that it isn't going to bite us in extended production environments... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
