Re: [PATCH -v5 0/9] migrate_pages(): batch TLB flushing

[Hugh Dickins <hughd@google.com>]

On Mon, 13 Feb 2023, Huang Ying wrote:

> From: "Huang, Ying" <ying.huang@intel.com>
> 
> Now, migrate_pages() migrate folios one by one, like the fake code as
> follows,
> 
>   for each folio
>     unmap
>     flush TLB
>     copy
>     restore map
> 
> If multiple folios are passed to migrate_pages(), there are
> opportunities to batch the TLB flushing and copying.  That is, we can
> change the code to something as follows,
> 
>   for each folio
>     unmap
>   for each folio
>     flush TLB
>   for each folio
>     copy
>   for each folio
>     restore map
> 
> The total number of TLB flushing IPI can be reduced considerably.  And
> we may use some hardware accelerator such as DSA to accelerate the
> folio copying.
> 
> So in this patch, we refactor the migrate_pages() implementation and
> implement the TLB flushing batching.  Base on this, hardware
> accelerated folio copying can be implemented.
> 
> If too many folios are passed to migrate_pages(), in the naive batched
> implementation, we may unmap too many folios at the same time.  The
> possibility for a task to wait for the migrated folios to be mapped
> again increases.  So the latency may be hurt.  To deal with this
> issue, the max number of folios be unmapped in batch is restricted to
> no more than HPAGE_PMD_NR in the unit of page.  That is, the influence
> is at the same level of THP migration.
> 
> We use the following test to measure the performance impact of the
> patchset,
> 
> On a 2-socket Intel server,
> 
>  - Run pmbench memory accessing benchmark
> 
>  - Run `migratepages` to migrate pages of pmbench between node 0 and
>    node 1 back and forth.
> 
> With the patch, the TLB flushing IPI reduces 99.1% during the test and
> the number of pages migrated successfully per second increases 291.7%.
> 
> Xin Hao helped to test the patchset on an ARM64 server with 128 cores,
> 2 NUMA nodes.  Test results show that the page migration performance
> increases up to 78%.
> 
> This patchset is based on mm-unstable 2023-02-10.

And back in linux-next this week: I tried next-20230217 overnight.

There is a deadlock in this patchset (and in previous versions: sorry
it's taken me so long to report), but I think one that's easily solved.

I've not bisected to precisely which patch (load can take several hours
to hit the deadlock), but it doesn't really matter, and I expect that
you can guess.

My root and home filesystems are ext4 (4kB blocks with 4kB PAGE_SIZE),
and so is the filesystem I'm testing, ext4 on /dev/loop0 on tmpfs.
So, plenty of ext4 page cache and buffer_heads.

Again and again, the deadlock is seen with buffer_migrate_folio_norefs(),
either in kcompactd0 or in khugepaged trying to compact, or in both:
it ends up calling __lock_buffer(), and that schedules away, waiting
forever to get BH_lock.  I have not identified who is holding BH_lock,
but I imagine a jbd2 journalling thread, and presume that it wants one
of the folio locks which migrate_pages_batch() is already holding; or
maybe it's all more convoluted than that.  Other tasks then back up
waiting on those folio locks held in the batch.

Never a problem with buffer_migrate_folio(), always with the "more
careful" buffer_migrate_folio_norefs().  And the patch below fixes
it for me: I've had enough hours with it now, on enough occasions,
to be confident of that.

Cc'ing Jan Kara, who knows buffer_migrate_folio_norefs() and jbd2
very well, and I hope can assure us that there is an understandable
deadlock here, from holding several random folio locks, then trying
to lock buffers.  Cc'ing fsdevel, because there's a risk that mm
folk think something is safe, when it's not sufficient to cope with
the diversity of filesystems.  I hope nothing more than the below is
needed (and I've had no other problems with the patchset: good job),
but cannot be sure.

[PATCH next] migrate_pages: fix deadlock on buffer heads

When __buffer_migrate_folio() is called from buffer_migrate_folio_norefs(),
force MIGRATE_ASYNC mode so that buffer_migrate_lock_buffers() will only
trylock_buffer(), failing with -EAGAIN as usual if that does not succeed.

Signed-off-by: Hugh Dickins <hughd@google.com>

--- next-20230217/mm/migrate.c
+++ fixed/mm/migrate.c
@@ -748,7 +748,8 @@ static int __buffer_migrate_folio(struct
 	if (folio_ref_count(src) != expected_count)
 		return -EAGAIN;
 
-	if (!buffer_migrate_lock_buffers(head, mode))
+	if (!buffer_migrate_lock_buffers(head,
+			check_refs ? MIGRATE_ASYNC : mode))
 		return -EAGAIN;
 
 	if (check_refs) {