On Sun, Feb 11, 2018 at 04:39:07PM +0800, Huang, Ying wrote:
> Hi, Russell,
>
> Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> writes:
>
> > On Sun, Feb 11, 2018 at 02:43:39PM +0800, Huang, Ying wrote:
> [snip]
> >>
> >>
> >> if page is an anonymous page in swap cache, "mapping &&
> >> !mapping_mapped()" will be true, so we will delay flushing. But if my
> >> understanding of the code were correct, we should call
> >> flush_kernel_dcache() because the kernel may access the page during
> >> swapping in/out.
> >>
> >> The code in other architectures follow the similar logic. Would it be
> >> better for page_mapping() here to return NULL for anonymous pages even
> >> if they are in swap cache? Of course we need to change the function
> >> name. page_file_mapping() appears a good name, but that has been used
> >> already. Any suggestion?
> >
> > flush_dcache_page() does nothing for anonymous pages (see cachetlb.txt,
> > it's only defined to do anything for page cache pages.)
> >
> > flush_anon_page() deals with anonymous pages.
>
> Thanks for your information! But I found this isn't followed exactly in
> the code. For example, in get_mergeable_page() in mm/ksm.c,
>
> if (PageAnon(page)) {
> flush_anon_page(vma, page, addr);
> flush_dcache_page(page);
> } else {
> put_page(page);
>
> flush_dcache_page() is called for anonymous pages too.
... and flush_dcache_page() will be a no-op here, as per its
documentation. Any flushing required here will have been taken care of
with flush_anon_page().
If flush_dcache_page() were to do flushing here, it would repeat the
flushing that flush_anon_page() has just done, so its pointless.
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