Re: isolate_lru_page on !head pages

[Michal Hocko]

On Mon 14-12-15 14:04:56, Kirill A. Shutemov wrote:
> On Wed, Dec 09, 2015 at 02:02:05PM +0100, Michal Hocko wrote:
> > Hi Kirill,
> 
> [ sorry for late reply, just back from vacation. ]
> 
> > while looking at the issue reported by Minchan [1] I have noticed that
> > there is nothing to prevent from "isolating" a tail page from LRU because
> > isolate_lru_page checks PageLRU which is
> > PAGEFLAG(LRU, lru, PF_HEAD)
> > so it is checked on the head page rather than the given page directly
> > but the rest of the operation is done on the given (tail) page.
> 
> Looks like most (all?) callers already exclude PTE-mapped THP already one
> way or another.

I can see e.g. do_move_page_to_node_array not doing a similar thing. It
isolates and then migrates potentially a tail page. I haven't looked
closer whether there is other hand break on the way though. The
point I was trying to make is that this is really _subtle_. We are
changing something else than we operate later on.

> Probably, VM_BUG_ON_PAGE(PageTail(page), page) in isolate_lru_page() would
> be appropriate.
> 
> > This is really subtle because this expects that every caller of this
> > function checks for the tail page otherwise we would clobber statistics
> > and who knows what else (I haven't checked that in detail) as the page
> > cannot be on the LRU list and the operation makes sense only on the head
> > page.
> > 
> > Would it make more sense to make PageLRU PF_ANY? That would return
> > false for PageLRU on any tail page and so it would be ignored by
> > isolate_lru_page.
> 
> I don't think this is right way to go. What we put on LRU is compound
> page, not 4k subpages. PageLRU() should return true if the compound page
> is on LRU regardless if you ask for head or tail page.

Hmm, but then we should operate on the head page because that is what
PageLRU operated on, no?

 
> False-negatives PageLRU() can be as bad as bug Minchan reported, but
> perhaps more silent.

-- 
Michal Hocko
SUSE Labs
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