Fix zfs_write() / mmap update_time() lock inversion #7942
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module/zfs/zfs_vnops.c
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| */ | ||
| if (flags == I_DIRTY_TIME) { | ||
| if (flags & I_DIRTY_TIME & |
module/zfs/zfs_vnops.c
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| * cannot be assigned set z_atime_dirty=1 so at least the times will | ||
| * be updated when the file is closed. | ||
| */ | ||
| boolean_t waited = B_FALSE; |
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I'm not sure I understand this. Is this initialization going to take effect once or every time?
We should move this to beginning of the function.
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Yes, of course thank you!
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| * cannot be assigned set z_atime_dirty=1 so at least the times will | ||
| * be updated when the file is closed. | ||
| */ | ||
| error = dmu_tx_assign(tx, (waited ? TXG_NOTHROTTLE : 0) | TXG_NOWAIT); |
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In dmu_tx_try_assign:
if (dn->dn_assigned_txg == tx->tx_txg - 1) {
mutex_exit(&dn->dn_mtx);
tx->tx_needassign_txh = txh;
DMU_TX_STAT_BUMP(dmu_tx_group);
return (SET_ERROR(ERESTART));
}
It doesn't seem that TXG_NOTHROTTLE will prevent dmu_tx_assign from failing? Perhaps we should bail out on waited && error == ERESTART?
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Good idea. You're right, it won't prevent it from failing under all circumstances, only due to the write throttle. I assume your specific concern is that we could end up spinning here, which would be just as bad.
| dmu_tx_abort(tx); | ||
| goto out; | ||
| zp->z_atime_dirty = 1; |
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The problem with this is that we only do lazy update atime, but not mtime and ctime. We need to add that otherwise we will lose those.
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That's true, unfortunate we really don't have a mechanism for that currently. Deferring the atime update to zfs_inactive already isn't really correct, but there was already machinery which I figured was better than nothing for this unlikely case. In order to handle all cases here I think we'd need to redirty the inode.
Codecov Report
@@ Coverage Diff @@
## master #7942 +/- ##
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+ Coverage 43.95% 43.96% +0.01%
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Files 318 318
Lines 103561 103566 +5
==========================================
+ Hits 45521 45535 +14
+ Misses 58040 58031 -9
Continue to review full report at Codecov.
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When a page is faulted in by filemap_page_mkwrite() this function may be called by update_time() with the file's mmap_sem held. Therefore it's necessary to use TXG_NOWAIT since we cannot release the mmap_sem, and even if we could, it would be undesirable to delay the page fault. TXG_NOTHROTTLE will be set as needed to bypass the write throttle. In the unlikely case the transaction cannot be assigned set z_atime_dirty=1 so at least the times will be updated when the file is closed. Signed-off-by: Brian Behlendorf <[email protected]>
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Closing in favor of the proposal in #7939. |
behlendorf
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Status: Abandoned
and removed
Status: Code Review Needed
Motivation and Context
Alternate approach to #7939, which is intended to address #7512.
Pushed for the purposes and review feedback, testing, and letting
the CI test it out.
Description
When a page is faulted in by filemap_page_mkwrite() this function
may be called by update_time() with the file's mmap_sem held.
Therefore it's necessary to use TXG_NOWAIT since we cannot release
the mmap_sem, and even if we could, it would be undesirable to
delay the page fault. TXG_NOTHROTTLE will be set as needed to
bypass the write throttle. In the unlikely case the transaction
cannot be assigned set z_atime_dirty=1 so at least the times will
be updated when the file is closed.
How Has This Been Tested?
Locally built and tested using the reproduced provided in #7939.
Types of changes
Checklist:
Signed-off-by.