Re: [PATCH] mm/hugetlb: simplify refs in memfd_alloc_folio

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On 9/4/2024 4:11 PM, Matthew Wilcox wrote:
On Wed, Sep 04, 2024 at 09:02:59PM +0100, Matthew Wilcox wrote:
On Wed, Sep 04, 2024 at 12:41:08PM -0700, Steve Sistare wrote:
The folio_try_get in memfd_alloc_folio is not necessary.  Delete it, and
delete the matching folio_put in memfd_pin_folios.  This also avoids
leaking a ref if the memfd_alloc_folio call to hugetlb_add_to_page_cache
fails, which would otherwise need an additional folio_put.  This is a
continuation of the fix
   "mm/hugetlb: fix memfd_pin_folios free_huge_pages leak"

I think you're right, but don't we also need to get rid of the
folio_put() call in the 'if (err)' case after calling
hugetlb_add_to_page_cache()?

After scratching my head about this a bit more, I was trying to preserve
the existing semantics of the code, but I think the code was always
buggy.

The correct code would be:

	folio = alloc_hugetlb_folio_nodemask(...);
	folio_put(folio);

The code as in tree today would trip an assertion:

         VM_BUG_ON_FOLIO(folio_ref_count(folio), folio);

as alloc_hugetlb_folio_nodemask() returns a folio with a refcount 1,
folio_try_get() would increment it to 2, folio_put() would decrement
it to 1, and so we'd call free_huge_folio() with a refcount of 1.

But after your patch, the code _is_ still wrong because we'll
start with a refcount of 1, fail to add to the page cache, call
folio_put() which will decrement the refcount to 0 _and call
free_huge_folio() itself_.  Then we'll call free_huge_folio()
on an already freed and possibly reallocated folio.

Indeed.  The explicit call to free_huge_folio must be deleted, as you
coded below.  I'll send a V2 of the patch.

- Steve

So every version suggested so far (current, yours, mine) is wrong,
and the right code looks like:

                 folio = alloc_hugetlb_folio_nodemask(...);
		if (folio) {
			err = hugetlb_add_to_page_cache(...);
			if (err) {
				folio_put(folio);
				return ERR_PTR(err);
			}
...

Or have I got something wrong in that analysis?

Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")

Suggested-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
Signed-off-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
---
  mm/gup.c   | 4 +---
  mm/memfd.c | 2 +-
  2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index bccabaa..947881ff 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -3618,7 +3618,7 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
  	pgoff_t start_idx, end_idx, next_idx;
  	struct folio *folio = NULL;
  	struct folio_batch fbatch;
-	struct hstate *h = NULL;
+	struct hstate *h;
  	long ret = -EINVAL;
if (start < 0 || start > end || !max_folios)
@@ -3662,8 +3662,6 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
  							     &fbatch);
  			if (folio) {
  				folio_put(folio);
-				if (h)
-					folio_put(folio);
  				folio = NULL;
  			}
diff --git a/mm/memfd.c b/mm/memfd.c
index bcb131d..f715301 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -89,7 +89,7 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
  						    numa_node_id(),
  						    NULL,
  						    gfp_mask);
-		if (folio && folio_try_get(folio)) {
+		if (folio) {
  			err = hugetlb_add_to_page_cache(folio,
  							memfd->f_mapping,
  							idx);
--
1.8.3.1






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