Re: [PATCH v2] mm, hugetlb: set PageLRU for in-use/active hugepages

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 17 Feb 2015 09:32:08 +0000 Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> wrote:

> Currently we are not safe from concurrent calls of isolate_huge_page(),
> which can make the victim hugepage in invalid state and results in BUG_ON().
> 
> The root problem of this is that we don't have any information on struct page
> (so easily accessible) about the hugepage's activeness. Note that hugepages'
> activeness means just being linked to hstate->hugepage_activelist, which is
> not the same as normal pages' activeness represented by PageActive flag.
> 
> Normal pages are isolated by isolate_lru_page() which prechecks PageLRU before
> isolation, so let's do similarly for hugetlb. PageLRU is unused on hugetlb now,
> so the change is mostly just inserting Set/ClearPageLRU (no conflict with
> current usage.) And the other changes are justified like below:
> - __put_compound_page() calls __page_cache_release() to do some LRU works,
>   but this is obviously for thps and assumes that hugetlb has always !PageLRU.
>   This assumption is not true any more, so this patch simply adds if (!PageHuge)
>   to avoid calling __page_cache_release() for hugetlb.
> - soft_offline_huge_page() now just calls list_move(), but generally callers
>   of page migration should use the common routine in isolation, so let's
>   replace the list_move() with isolate_huge_page() rather than inserting
>   ClearPageLRU.
> 
> Set/ClearPageLRU should be called within hugetlb_lock, but hugetlb_cow() and
> hugetlb_no_page() don't do this. This is justified because in these function
> SetPageLRU is called right after the hugepage is allocated and no other thread
> tries to isolate it.

Whoa.

So if I'm understanding this correctly, hugepages never have PG_lru set
and so you are overloading that bit on hugepages to indicate that the
page is present on hstate->hugepage_activelist?

This is somewhat of a big deal and the patch doesn't make it very clear
at all.  We should

- document PG_lru, for both of its identities

- consider adding a new PG_hugepage_active(?) flag which has the same
  value as PG_lru (see how PG_savepinned was done).

- create suitable helper functions for the new PG_lru meaning. 
  Simply calling PageLRU/SetPageLRU for pages which *aren't on the LRU*
  is lazy and misleading.  Create a name for the new concept
  (hugepage_active?) and document it and use it consistently.


> @@ -75,7 +76,8 @@ static void __put_compound_page(struct page *page)
>  {
>  	compound_page_dtor *dtor;
>  
> -	__page_cache_release(page);
> +	if (!PageHuge(page))
> +		__page_cache_release(page);
>  	dtor = get_compound_page_dtor(page);
>  	(*dtor)(page);

And this needs a good comment - there's no way that a reader can work
out why this code is here unless he goes dumpster diving in the git
history.

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