Re: [RFC PATCH] migrate_pages: Never block waiting for the page lock

[Huang, Ying]

Douglas Anderson <dianders@xxxxxxxxxxxx> writes:

> Currently when we try to do page migration and we're in "synchronous"
> mode (and not doing direct compaction) then we'll wait an infinite
> amount of time for a page lock. This does not appear to be a great
> idea.
>
> One issue can be seen when I put a device under extreme memory
> pressure. I took a sc7180-trogdor Chromebook (4GB RAM, 8GB zram
> swap). I ran the browser along with Android (which runs from a
> loopback mounted 128K block-size squashfs "disk"). I then manually ran
> the mmm_donut memory pressure tool [1]. The system is completely
> unusable both with and without this patch since there are 8 processes
> completely thrashing memory, but it was still interesting to look at
> how migration was behaving. I put some timing code in and I could see
> that we sometimes waited over 25 seconds (in the context of
> kcompactd0) for a page lock to become available. Although the 25
> seconds was the high mark, it was easy to see tens, hundreds, or
> thousands of milliseconds spent waiting on the lock.
>
> Instead of waiting, if I bailed out right away (as this patch does), I
> could see kcompactd0 move forward to successfully to migrate other
> pages instead. This seems like a better use of kcompactd's time.
>
> Thus, even though this didn't make the system any more usable in my
> absurd test case, it still seemed to make migration behave better and
> that feels like a win. It also makes the code simpler since we have
> one fewer special case.

TBH, the test case is too extreme for me.

And, we have multiple "sync" mode to deal with latency requirement, for
example, we use MIGRATE_SYNC_LIGHT for compaction to avoid too long
latency.  If you have latency requirement for some users, you may
consider to add new "sync" mode.

Best Regards,
Huang, Ying

> The second issue that this patch attempts to fix is one that I haven't
> managed to reproduce yet. We have crash reports from the field that
> report that kcompactd0 was blocked for more than ~120 seconds on this
> exact lock. These crash reports are on devices running older kernels
> (5.10 mostly). In most of these crash reports the device is under
> memory pressure and many tasks were blocked in squashfs code, ext4
> code, or memory allocation code. While I don't know if unblocking
> kcompactd would actually have helped relieve the memory pressure, at
> least there was a chance that it could have helped a little bit.
>
> [1] https://chromium.googlesource.com/chromiumos/platform/microbenchmarks/+/refs/heads/main/mmm_donut.py
>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
>
>  mm/migrate.c | 25 +++++++------------------
>  1 file changed, 7 insertions(+), 18 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index db3f154446af..dfb0a6944181 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1143,26 +1143,15 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
>  	dst->private = NULL;
>  
>  	if (!folio_trylock(src)) {
> -		if (mode == MIGRATE_ASYNC)
> -			goto out;
> -
>  		/*
> -		 * It's not safe for direct compaction to call lock_page.
> -		 * For example, during page readahead pages are added locked
> -		 * to the LRU. Later, when the IO completes the pages are
> -		 * marked uptodate and unlocked. However, the queueing
> -		 * could be merging multiple pages for one bio (e.g.
> -		 * mpage_readahead). If an allocation happens for the
> -		 * second or third page, the process can end up locking
> -		 * the same page twice and deadlocking. Rather than
> -		 * trying to be clever about what pages can be locked,
> -		 * avoid the use of lock_page for direct compaction
> -		 * altogether.
> +		 * While there are some modes we could be running in where we
> +		 * could block here waiting for the lock (specifically
> +		 * modes other than MIGRATE_ASYNC and when we're not in
> +		 * direct compaction), it's not worth the wait. Instead of
> +		 * waiting, we'll bail. This will let the caller try to
> +		 * migrate some other pages that aren't contended.
>  		 */
> -		if (current->flags & PF_MEMALLOC)
> -			goto out;
> -
> -		folio_lock(src);
> +		goto out;
>  	}
>  	locked = true;