Re: [PATCH] mm/gup: Drain batched mlock folio processing before attempting migration

[Hugh Dickins <hughd@google.com>]

On Thu, 28 Aug 2025, David Hildenbrand wrote:
> On 28.08.25 18:12, Hugh Dickins wrote:
> > On Thu, 28 Aug 2025, David Hildenbrand wrote:
> >> On 28.08.25 10:47, Hugh Dickins wrote:
> > ...
> >>> It took several days in search of the least bad compromise, but
> >>> in the end I concluded the opposite of what we'd intended above.
> >>>
> >>> There is a fundamental incompatibility between my 5.18 2fbb0c10d1e8
> >>> ("mm/munlock: mlock_page() munlock_page() batch by pagevec")
> >>> and Ge Yang's 6.11 33dfe9204f29
> >>> ("mm/gup: clear the LRU flag of a page before adding to LRU batch").
> >>>
> >>> It turns out that the mm/swap.c folio batches (apart from lru_add)
> >>> are all for best-effort, doesn't matter if it's missed, operations;
> >>> whereas mlock and munlock are more serious.  Probably mlock could
> >>> be (not very satisfactorily) converted, but then munlock?  Because
> >>> of failed folio_test_clear_lru()s, it would be far too likely to
> >>> err on either side, munlocking too soon or too late.
> >>>
> >>> I've concluded that one or the other has to go.  If we're having
> >>> a beauty contest, there's no doubt that 33dfe9204f29 is much nicer
> >>> than 2fbb0c10d1e8 (which is itself far from perfect).  But functionally,
> >>> I'm afraid that removing the mlock/munlock batching will show up as a
> >>> perceptible regression in realistic workloadsg; and on consideration,
> >>> I've found no real justification for the LRU flag clearing change.
> >>
> >> Just to understand what you are saying: are you saying that we will go back
> >> to
> >> having a folio being part of multiple LRU caches?
> > 
> > Yes.  Well, if you count the mlock/munlock batches in as "LRU caches",
> > then that has been so all along.
> 
> Yes ...
> 
> > 
> >> :/ If so, I really rally
> >> hope that we can find another way and not go back to that old handling.
> > 
> > For what reason?  It sounded like a nice "invariant" to keep in mind,
> > but it's a limitation, and  what purpose was it actually serving?
> 
> I liked the semantics that if !lru, there could be at most one reference from
> the LRU caches.
> 
> That is, if there are two references, you don't even have to bother with
> flushing anything.

If that assumption is being put into practice anywhere (not that I know of),
then it's currently wrong and needs currecting.

It would be nice and simple if it worked, but I couldn't get it to work
with mlock/munlock batching, so it seemed better to give up on the
pretence.

And one of the points of using a pagevec used to be precisely that a
page could exist on more than one at a time (unlike threading via lru).

> 
> > 
> > If it's the "spare room in struct page to keep the address of that one
> > batch entry ... efficiently extract ..." that I was dreaming of: that
> > has to be a future thing, when perhaps memdescs will allow an extensible
> > structure to be attached, and extending it for an mlocked folio (to hold
> > the mlock_count instead of squeezing it into lru.prev) would not need
> > mlock/munlock batching at all (I guess: far from uppermost in my mind!),
> > and including a field for "efficiently extract" from LRU batch would be
> > nice.
> > 
> > But, memdescs or not, there will always be pressure to keep the common
> > struct as small as possible, so I don't know if we would actually go
> > that way.
> > 
> > But I suspect that was not your reason at all: please illuminate.
> 
> You are very right :)

OK, thanks, I'll stop reading further now :)

> 
> Regarding the issue at hand:
> 
> There were discussions at LSF/MM about also putting (some) large folios onto
> the LRU caches.
> 
> In that context, GUP could take multiple references on the same folio, and a
> simple folio_expected_ref_count() + 1 would no longer do the trick.
> 
> I thought about this today, and likely it could be handled by scanning the
> page array for same folios etc. A bit nasty when wanting to cover all corner
> cases (folio pages must not be consecutive in the passed array ) ...

I haven't thought about that problem at all (except when working around
a similar issue in mm/mempolicy.c's folio queueing), but can sympathize.

It had irritated me to notice how the flush-immediately code for 512-page
folios now extends to 2-page folios: you've enlightened me why that remains
so, I hadn't thought of the implications.  Perhaps even more reason to
allow for multiple references on the pagevecs/batches?

> 
> Apart from that issue, I liked the idea of a "single entry in the cache" for
> other reasons: it gives clear semantics. We cannot end up in a scenario where
> someone performs OPX and later someone OPY on a folio, but the way the lru
> caches are flushed we might end up processing OPX after OPY -- this should be
> able to happen in case of local or remote flushes IIRC.

It's been that way for many years, that's how they are.

> 
> Anyhow, I quickly scanned your code. The folio_expected_ref_count() should
> solve the issue for now. It's quite unfortunate that any raised reference will
> make us now flush all remote LRU caches.

There will be more false positives (drains which drain nothing relevant)
by relying on ref_count rather than test_lru, yes.  But certainly there
were already false positives by relying on test_lru.  And I expect the
preparatory local lru_add_drain() to cut out a lot of true positives.

> 
> Maybe we just want to limit it to !folio_test_large(), because flushing there
> really doesn't give us any chance in succeeding right now? Not sure if it
> makes any difference in practice, though, we'll likely be flushing remote LRU
> caches now more frequently either way.

Ah, good idea. with or without my changes.  Maybe material for a separate
patch.  I wonder if we would do better to add a folio_is_batchable() and
use that consistently in all of the places which are refusing to batch
when folio_test_large() - I wonder if a !folio_test_large() here will
get missed if there's a change there.

Thanks,
Hugh