| Message ID | 20240422194217.442933-1-david@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation | expand |
On 2024/4/23 03:42, David Hildenbrand wrote: > The documentation is wrong and relying on it almost resulted in BUGs in > new callers: ever since fd4a7ac32918 ("mm: migrate: try again > if THP split is failed due to page refcnt") we return -EAGAIN on > unexpected folio references, not -EBUSY. > > Let's fix that and also document which other return values we can > currently see and why they could happen. > > Reviewed-by: Zi Yan <ziy@nvidia.com> > Reviewed-by: John Hubbard <jhubbard@nvidia.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: John Hubbard <jhubbard@nvidia.com> > Cc: Matthew Wilcox <willy@infradead.org> > Cc: Baolin Wang <baolin.wang@linux.alibaba.com> > Signed-off-by: David Hildenbrand <david@redhat.com> > --- > > v1 -> v2: > * Also document concurrent removal from the page cache (likely we should > return -EBUSY -- but likely it doesn't really matter). > * Reference fd4a7ac32918 in patch description > > --- > mm/huge_memory.c | 14 +++++++++++--- > 1 file changed, 11 insertions(+), 3 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index ee12726291f1b..a7406267323ed 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -2956,7 +2956,7 @@ bool can_split_folio(struct folio *folio, int *pextra_pins) > * > * 3) The folio must not be pinned. Any unexpected folio references, including > * GUP pins, will result in the folio not getting split; instead, the caller > - * will receive an -EBUSY. > + * will receive an -EAGAIN. > * > * 4) @new_order > 1, usually. Splitting to order-1 anonymous folios is not > * supported for non-file-backed folios, because folio->_deferred_list, which > @@ -2975,8 +2975,16 @@ bool can_split_folio(struct folio *folio, int *pextra_pins) > * > * Returns 0 if the huge page was split successfully. > * > - * Returns -EBUSY if @page's folio is pinned, or if the anon_vma disappeared > - * from under us. > + * Returns -EAGAIN if the folio has unexpected reference (e.g., GUP) or if > + * the folio was concurrently removed from the page cache. > + * > + * Returns -EBUSY when trying to split the huge zeropage, if the folio is > + * under writeback, if fs-specific folio metadata cannot currently be > + * released, or if some unexpected race happened (e.g., anon VMA disappeared, > + * truncation). > + * > + * Returns -EINVAL when trying to split to an order that is incompatible > + * with the folio. Splitting to order 0 is compatible with all folios. > */ > int split_huge_page_to_list_to_order(struct page *page, struct list_head *list, > unsigned int new_order) LGTM. Thanks for fixing the document that I missed:) Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index ee12726291f1b..a7406267323ed 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2956,7 +2956,7 @@ bool can_split_folio(struct folio *folio, int *pextra_pins) * * 3) The folio must not be pinned. Any unexpected folio references, including * GUP pins, will result in the folio not getting split; instead, the caller - * will receive an -EBUSY. + * will receive an -EAGAIN. * * 4) @new_order > 1, usually. Splitting to order-1 anonymous folios is not * supported for non-file-backed folios, because folio->_deferred_list, which @@ -2975,8 +2975,16 @@ bool can_split_folio(struct folio *folio, int *pextra_pins) * * Returns 0 if the huge page was split successfully. * - * Returns -EBUSY if @page's folio is pinned, or if the anon_vma disappeared - * from under us. + * Returns -EAGAIN if the folio has unexpected reference (e.g., GUP) or if + * the folio was concurrently removed from the page cache. + * + * Returns -EBUSY when trying to split the huge zeropage, if the folio is + * under writeback, if fs-specific folio metadata cannot currently be + * released, or if some unexpected race happened (e.g., anon VMA disappeared, + * truncation). + * + * Returns -EINVAL when trying to split to an order that is incompatible + * with the folio. Splitting to order 0 is compatible with all folios. */ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list, unsigned int new_order)
The documentation is wrong and relying on it almost resulted in BUGs in new callers: ever since fd4a7ac32918 ("mm: migrate: try again if THP split is failed due to page refcnt") we return -EAGAIN on unexpected folio references, not -EBUSY. Let's fix that and also document which other return values we can currently see and why they could happen. Reviewed-by: Zi Yan <ziy@nvidia.com> Reviewed-by: John Hubbard <jhubbard@nvidia.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: John Hubbard <jhubbard@nvidia.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Baolin Wang <baolin.wang@linux.alibaba.com> Signed-off-by: David Hildenbrand <david@redhat.com> --- v1 -> v2: * Also document concurrent removal from the page cache (likely we should return -EBUSY -- but likely it doesn't really matter). * Reference fd4a7ac32918 in patch description --- mm/huge_memory.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-)