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
