Re: [PATCH 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free

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On 26.02.24 13:57, Ryan Roberts wrote:
On 26/02/2024 08:35, Lance Yang wrote:
Hey Fengwei,

Thanks for taking time to review!

On Mon, Feb 26, 2024 at 10:38 AM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
On Sun, Feb 25, 2024 at 8:32 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
[...]
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -676,11 +676,43 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
                */
               if (folio_test_large(folio)) {
                       int err;
+                     unsigned long next_addr, align;

-                     if (folio_estimated_sharers(folio) != 1)
-                             break;
-                     if (!folio_trylock(folio))
-                             break;
+                     if (folio_estimated_sharers(folio) != 1 ||
+                         !folio_trylock(folio))
+                             goto skip_large_folio;
+
+                     align = folio_nr_pages(folio) * PAGE_SIZE;
+                     next_addr = ALIGN_DOWN(addr + align, align);
There is a possible corner case:
If there is a cow folio associated with this folio and the cow folio
has smaller size than this folio for whatever reason, this change can't
handle it correctly.

Thanks for pointing that out; it's very helpful to me!
I made some changes. Could you please check if this corner case is now resolved?

As a diff against this patch.

diff --git a/mm/madvise.c b/mm/madvise.c
index bcbf56595a2e..c7aacc9f9536 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -686,10 +686,12 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
  			next_addr = ALIGN_DOWN(addr + align, align);
/*
-			 * If we mark only the subpages as lazyfree,
-			 * split the large folio.
+			 * If we mark only the subpages as lazyfree, or
+			 * if there is a cow folio associated with this folio,
+			 * then split the large folio.
  			 */
-			if (next_addr > end || next_addr - addr != align)
+			if (next_addr > end || next_addr - addr != align ||
+			    folio_total_mapcount(folio) != folio_nr_pages(folio))

I still don't think this is correct. I think you were previously assuming that
if you see a page from a large folio then the whole large folio should be
contiguously mapped? This new check doesn't validate that assumption reliably;
you need to iterate through every pte to generate a batch, like David does in
folio_pte_batch() for this to be safe.

An example of when this check is insufficient; let's say you have a 4 page anon
folio mapped contiguously in a process (total_mapcount=4). The process is forked
(total_mapcount=8). Then each process munmaps the second 2 pages
(total_mapcount=4). In place of the munmapped 2 pages, 2 new pages are mapped.
Then call madvise. It's probably even easier to trigger for file-backed memory
(I think this code path is used for both file and anon?)

What would work here is using folio_pte_batch() to get how many PTEs are mapped *here*, then comparing the the batch size to folio_nr_pages(). If both match, we are mapping all subpages.

--
Cheers,

David / dhildenb





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