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SubjectRe: [PATCH 1/2] mm: Remove redundant test from find_get_pages_contig
On Fri, Nov 23, 2018 at 01:47:32PM +0300, Kirill A. Shutemov wrote:
> On Thu, Nov 22, 2018 at 01:32:23PM -0800, Matthew Wilcox wrote:
> > After we establish a reference on the page, we check the pointer continues
> > to be in the correct position in i_pages. There's no need to check the
> > page->mapping or page->index afterwards; if those can change after we've
> > got the reference, they can change after we return the page to the caller.
>
> Hm. IIRC, page->mapping can be set to NULL due truncation, but what about
> index? When it can be changed? Truncation doesn't touch it.

I think index can only be changed after the refcount has hit zero and
the page is safely out of the pagecache. I agree that page->mapping can
be set to NULL after the call to xas_reload() ... but then it can also
happen after the check, so the check isn't really buying us anything
that the xas_reload() call doesn't already check.

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