linux-kernel - Re: [PATCH 1/2] mm: Remove redundant test from find_get_pages

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Date:   Fri, 23 Nov 2018 09:59:32 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [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.