Re: [PATCH v7 19/20] fs/dax: Properly refcount fs dax pages
flat view
From: Guenter Roeck <linux@roeck-us.net>
Date: 2025-02-06 21:06:32
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On Wed, Feb 05, 2025 at 09:48:16AM +1100, Alistair Popple wrote:
Currently fs dax pages are considered free when the refcount drops to one and their refcounts are not increased when mapped via PTEs or decreased when unmapped. This requires special logic in mm paths to detect that these pages should not be properly refcounted, and to detect when the refcount drops to one instead of zero. On the other hand get_user_pages(), etc. will properly refcount fs dax pages by taking a reference and dropping it when the page is unpinned. Tracking this special behaviour requires extra PTE bits (eg. pte_devmap) and introduces rules that are potentially confusing and specific to FS DAX pages. To fix this, and to possibly allow removal of the special PTE bits in future, convert the fs dax page refcounts to be zero based and instead take a reference on the page each time it is mapped as is currently the case for normal pages. This may also allow a future clean-up to remove the pgmap refcounting that is currently done in mm/gup.c. Signed-off-by: Alistair Popple <apopple@nvidia.com> Reviewed-by: Dan Williams <redacted> ---
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quoted hunk ↗ jump to hunk
-static inline unsigned long dax_page_share_put(struct page *page) +static inline unsigned long dax_folio_put(struct folio *folio) { - WARN_ON_ONCE(!page->share); - return --page->share; + unsigned long ref; + + if (!dax_folio_is_shared(folio)) + ref = 0; + else + ref = --folio->share; + + WARN_ON_ONCE(ref < 0);
Kind of unlikely for an unsigned long to ever be < 0. [ thanks to coverity for noticing ] Guenter