Re: [PATCH] mm/rmap: do not add fully unmapped large folio to deferred split list
From: David Hildenbrand
Date: Mon Apr 15 2024 - 11:13:26 EST
On 12.04.24 23:06, Zi Yan wrote:
On 12 Apr 2024, at 15:32, David Hildenbrand wrote:
On 12.04.24 16:35, Zi Yan wrote:
On 11 Apr 2024, at 11:46, David Hildenbrand wrote:
On 11.04.24 17:32, Zi Yan wrote:
From: Zi Yan <ziy@xxxxxxxxxx>
In __folio_remove_rmap(), a large folio is added to deferred split list
if any page in a folio loses its final mapping. It is possible that
the folio is unmapped fully, but it is unnecessary to add the folio
to deferred split list at all. Fix it by checking folio mapcount before
adding a folio to deferred split list.
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
mm/rmap.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index 2608c40dffad..d599a772e282 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1494,7 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
enum rmap_level level)
{
atomic_t *mapped = &folio->_nr_pages_mapped;
- int last, nr = 0, nr_pmdmapped = 0;
+ int last, nr = 0, nr_pmdmapped = 0, mapcount = 0;
enum node_stat_item idx;
__folio_rmap_sanity_checks(folio, page, nr_pages, level);
@@ -1506,7 +1506,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
break;
}
- atomic_sub(nr_pages, &folio->_large_mapcount);
+ mapcount = atomic_sub_return(nr_pages,
+ &folio->_large_mapcount) + 1;
That becomes a new memory barrier on some archs. Rather just re-read it below. Re-reading should be fine here.
Would atomic_sub_return_relaxed() work? Originally I was using atomic_read(mapped)
below, but to save an atomic op, I chose to read mapcount here.
Some points:
(1) I suggest reading about atomic get/set vs. atomic RMW vs. atomic
RMW that return a value -- and how they interact with memory barriers.
Further, how relaxed variants are only optimized on some architectures.
atomic_read() is usually READ_ONCE(), which is just an "ordinary" memory
access that should not be refetched. Usually cheaper than most other stuff
that involves atomics.
I should have checked the actual implementation instead of being fooled
by the name. Will read about it. Thanks.
(2) We can either use folio_large_mapcount() == 0 or !atomic_read(mapped)
to figure out if the folio is now completely unmapped.
(3) There is one fundamental issue: if we are not batch-unmapping the whole
thing, we will still add the folios to the deferred split queue. Migration
would still do that, or if there are multiple VMAs covering a folio.
(4) We should really avoid making common operations slower only to make
some unreliable stats less unreliable.
We should likely do something like the following, which might even be a bit
faster in some cases because we avoid a function call in case we unmap
individual PTEs by checking _deferred_list ahead of time
diff --git a/mm/rmap.c b/mm/rmap.c
index 2608c40dffad..356598b3dc3c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
* page of the folio is unmapped and at least one page
* is still mapped.
*/
- if (folio_test_large(folio) && folio_test_anon(folio))
- if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
- deferred_split_folio(folio);
+ if (folio_test_large(folio) && folio_test_anon(folio) &&
+ (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped) &&
+ atomic_read(mapped) &&
+ data_race(list_empty(&folio->_deferred_list)))
data_race() might not be needed, as Ryan pointed out[1]
Right, I keep getting confused by that. Likely we should add data_race()
only if we get actual reports.
--
Cheers,
David / dhildenb