Re: [PATCH RFC] Ext4: fix deadlock on dirty pages between fault and writeback

Jan Kara
2018-11-30T03:04:26Z

On Thu 29-11-18 22:07:44, Zhengping Zhou wrote:
> in kenrel 4.20-rc1 , function shrink_page_list：
>
> 1227 if (PageWriteback(page)) {
> 1228 /* Case 1 above */
> 1229 if (current_is_kswapd() &&
> 1230 PageReclaim(page) &&
> 1231 test_bit(PGDAT_WRITEBACK, &pgdat->flags)) {
> 1232 nr_immediate++;
> 1233 goto activate_locked;
> 1234
> 1235 /* Case 2 above */
> 1236 } else if (sane_reclaim(sc) ||
> 1237 !PageReclaim(page) || !may_enter_fs) {
> 1238 /*
> 1239 * This is slightly racy - end_page_writeback()
> 1240 * might have just cleared PageReclaim, then
> 1241 * setting PageReclaim here end up interpreted
> 1242 * as PageReadahead - but that does not matter
> 1243 * enough to care. What we do want is for this
> 1244 * page to have PageReclaim set next time memcg
> 1245 * reclaim reaches the tests above, so it will
> 1246 * then wait_on_page_writeback() to avoid OOM;
> 1247 * and it's also appropriate in global reclaim.
> 1248 */
> 1249 SetPageReclaim(page);
> 1250 nr_writeback++;
> 1251 goto activate_locked;
> 1252
> 1253 /* Case 3 above */
> 1254 } else {
> 1255 unlock_page(page);
> 1256 wait_on_page_writeback(page);
> 1257 /* then go back and try same page again */
> 1258 list_add_tail(&page->lru, page_list);
> 1259 continue;
> 1260 }
> 1261 }
>
>
> What's your kernel version ? Seems we won't wait_page_writeback with
> holding page lock in latest kernel version.

The page lock we hold is the page lock that is already held when doing
memory allocation. So it is not the one that is acquired and released by
the page reclaim code.

Honza

> Jan Kara <jack@suse.cz> 于2018年11月29日周四 下午9:01写道：
> >
> > On Thu 29-11-18 23:02:53, Dave Chinner wrote:
> > > On Thu, Nov 29, 2018 at 09:52:38AM +0100, Jan Kara wrote:
> > > > On Wed 28-11-18 12:11:23, Liu Bo wrote:
> > > > > On Tue, Nov 27, 2018 at 12:42:49PM +0100, Jan Kara wrote:
> > > > > > CCed fsdevel since this may be interesting to other filesystem developers
> > > > > > as well.
> > > > > >
> > > > > > On Tue 30-10-18 08:22:49, Liu Bo wrote:
> > > > > > > mpage_prepare_extent_to_map() tries to build up a large bio to stuff down
> > > > > > > the pipe. But if it needs to wait for a page lock, it needs to make sure
> > > > > > > and send down any pending writes so we don't deadlock with anyone who has
> > > > > > > the page lock and is waiting for writeback of things inside the bio.
> > > > > >
> > > > > > Thanks for report! I agree the current code has a deadlock possibility you
> > > > > > describe. But I think the problem reaches a bit further than what your
> > > > > > patch fixes. The problem is with pages that are unlocked but have
> > > > > > PageWriteback set. Page reclaim may end up waiting for these pages and
> > > > > > thus any memory allocation with __GFP_FS set can block on these. So in our
> > > > > > current setting page writeback must not block on anything that can be held
> > > > > > while doing memory allocation with __GFP_FS set. Page lock is just one of
> > > > > > these possibilities, wait_on_page_writeback() in
> > > > > > mpage_prepare_extent_to_map() is another suspect and there mat be more. Or
> > > > > > to say it differently, if there's lock A and GFP_KERNEL allocation can
> > > > > > happen under lock A, then A cannot be taken by the writeback path. This is
> > > > > > actually pretty subtle deadlock possibility and our current lockdep
> > > > > > instrumentation isn't going to catch this.
> > > > > >
> > > > >
> > > > > Thanks for the nice summary, it's true that a lock A held in both
> > > > > writeback path and memory reclaim can end up with deadlock.
> > > > >
> > > > > Fortunately, by far there're only deadlock reports of page's lock bit
> > > > > and writeback bit in both ext4 and btrfs[1]. I think
> > > > > wait_on_page_writeback() would be OK as it's been protected by page
> > > > > lock.
> > > > >
> > > > > [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=01d658f2ca3c85c1ffb20b306e30d16197000ce7
> > > >
> > > > Yes, but that may just mean that the other deadlocks are just harder to
> > > > hit...
> > > >
> > > > > > So I see two ways how to fix this properly:
> > > > > >
> > > > > > 1) Change ext4 code to always submit the bio once we have a full page
> > > > > > prepared for writing. This may be relatively simple but has a higher CPU
> > > > > > overhead for bio allocation & freeing (actual IO won't really differ since
> > > > > > the plugging code should take care of merging the submitted bios). XFS
> > > > > > seems to be doing this.
> > > > >
> > > > > Seems that that's the safest way to do it, but as you said there's
> > > > > some tradeoff.
> > > > >
> > > > > (Just took a look at xfs's writepages, xfs also did the page
> > > > > collection if there're adjacent pages in xfs_add_to_ioend(), and since
> > > > > xfs_vm_writepages() is using the generic helper write_cache_pages()
> > > > > which calls lock_page() as well, it's still possible to run into the
> > > > > above kind of deadlock.)
> > > >
> > > > Originally I thought XFS doesn't have this problem but now when I look
> > > > again, you are right that their ioend may accumulate more pages to write
> > > > and so they are prone to the same deadlock ext4 is. Added XFS list to CC.
> > >
> > > I don't think XFS has a problem here, because the deadlock is
> > > dependent on holding a lock that writeback might take and then doing
> > > a GFP_KERNEL allocation. I don't think we do that anywhere in XFS -
> > > the only lock that is of concern here is the ip->i_ilock, and I
> > > think we always do GFP_NOFS allocations inside that lock.
> > >
> > > As it is, this sort of lock vs reclaim inversion should be caught by
> > > lockdep - allocations and reclaim contexts are recorded by lockdep
> > > we get reports if we do lock A - alloc and then do reclaim - lock A.
> > > We've always had problems with false positives from lockdep for
> > > these situations where common XFS code can be called from GFP_KERNEL
> > > valid contexts as well as reclaim or GFP_NOFS-only contexts, but I
> > > don't recall ever seeing such a report for the writeback path....
> >
> > I think for A == page lock, XFS may have the problem (and lockdep won't
> > notice because it does not track page locks). There are some parts of
> > kernel which do GFP_KERNEL allocations under page lock - pte_alloc_one() is
> > one such function which allocates page tables with GFP_KERNEL and gets
> > called with the faulted page locked. And I believe there are others.
> >
> > So direct reclaim from pte_alloc_one() can wait for writeback on page B
> > while holding lock on page A. And if B is just prepared (added to bio,
> > under writeback, unlocked) but not submitted in xfs_writepages() and we
> > block on lock_page(A), we have a deadlock.
> >
> > Generally deadlocks like these will be invisible to lockdep because it does
> > not track either PageWriteback or PageLocked as a dependency.
> >
> > > > > > 2) Change the code to unlock the page only when we submit the bio.
> > >
> > > > > This sounds doable but not good IMO, the concern is that page locks
> > > > > can be held for too long, or if we do 2), submitting one bio per page
> > > > > in 1) is also needed.
> > > >
> > > > Hum, you're right that page lock hold times may increase noticeably and
> > > > that's not very good. Ideally we'd need a way to submit whatever we have
> > > > prepared when we are going to sleep but there's no easy way to do that.
> > >
> > > XFS unlocks the page after it has been added to the bio and marked
> > > as under writeback, not when the bio is submitted. i.e.
> > >
> > > writepage w/ locked page dirty
> > > lock ilock
> > > <mapping, allocation>
> > > unlock ilock
> > > bio_add_page
> > > clear_page_dirty_for_io
> > > set_page_writeback
> > > unlock_page
> > > .....
> > > <gather more dirty pages into bio>
> > > .....
> > > <bio is full or discontiguous page to be written>
> > > submit_bio()
> >
> > Yes, ext4 works the same way. But thanks for confirmation.
> >
> > > If we switch away which holding a partially built bio, the only page
> > > we have locked is the one we are currently trying to add to the bio.
> > > Lock ordering prevents deadlocks on that one page, and all other
> > > pages in the bio being built are marked as under writeback and are
> > > not locked. Hence anything that wants to modify a page held in the
> > > bio will block waiting for page writeback to clear, not the page
> > > lock.
> >
> > Yes, and the blocking on writeback of such page in direct reclaim is
> > exactly one link in the deadlock chain...
> >
> > Honza
> > --
> > Jan Kara <jack@suse.com>
> > SUSE Labs, CR
--
Jan Kara <jack@suse.com>
SUSE Labs, CR