Re: [PATCH 2.6.17-rc1-mm3] add migratepage addresss space op to shmem

[Hugh Dickins <hugh@veritas.com>]

[Sorry, I seem to have deleted the original, so destroyed threading]

On Thu, 20 Apr 2006, Lee Schermerhorn wrote:
> 
> In 2.6.16 through 2.6.17-rc1, shared memory mappings do not
> have a migratepage address space op.  Therefore, migrate_pages()
> falls back to default processing.  In this path, it will try to
> pageout() dirty pages.  Once a shared memory page has been migrated
> it becomes dirty, so migrate_pages() will try to page it out.  
> However, because the page count is 3 [cache + current + pte],
> pageout() will return PAGE_KEEP because is_page_cache_freeable()
> returns false.  This will abort all subsequent migrations.

So far as I can see, this problem is not at all peculiar to shmem
(aside from its greater likelihood of being found PageDirty): won't
that PageDirty pageout in migrate_pages always return PAGE_KEEP?
so as it stands, is pointless and misleading?

> This patch adds a migratepage address space op to shared memory
> segments to avoid taking the default path.  We use the "migrate_page()"
> function because it knows how to migrate dirty pages.  This allows
> shared memory segment pages to migrate, subject to other conditions
> such as # pte's referencing the page [page_mapcount(page)], when
> requested.  

While that's not wrong, wouldn't the right fix be something else?

> I think this is safe.  If we're migrating a shared memory page,
> then we found the page via a page table, so it must be in
> memory.

Yes, I agree: the isolate_lru_page while holding ptl keeps it sane.

Hugh

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