isolate_lru_page on !head pages

isolate_lru_page on !head pages

[Michal Hocko]

Hi Kirill,
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.

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 haven't checked other flags but there might be a similar situation. I
am wondering whether it is really a good idea to perform a flag check on
a different page then the operation which depends on the result of the
test in general. It sounds like a maintenance horror to me.

[1] http://lkml.kernel.org/r/20151201133455.GB27574@bbox
-- 
Michal Hocko
SUSE Labs

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Re: isolate_lru_page on !head pages

[Kirill A. Shutemov]

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.
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.

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

> I haven't checked other flags but there might be a similar situation. I
> am wondering whether it is really a good idea to perform a flag check on
> a different page then the operation which depends on the result of the
> test in general. It sounds like a maintenance horror to me.
> 
> [1] http://lkml.kernel.org/r/20151201133455.GB27574@bbox
-- 
 Kirill A. Shutemov

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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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Re: isolate_lru_page on !head pages

[Kirill A. Shutemov]

On Tue, Dec 15, 2015 at 09:52:33AM +0100, Michal Hocko wrote:
> 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.

No, it doesn't. follow_page(FOLL_SPLIT) would split THP pages.

> 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?

head page is what linked into LRU, but not nessesary the way we obtain the
page to check. If we check PageLRU(pte_page(*pte)) it should produce the
right result.

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

-- 
 Kirill A. Shutemov

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Re: isolate_lru_page on !head pages

[Michal Hocko]

On Tue 15-12-15 14:03:18, Kirill A. Shutemov wrote:
> On Tue, Dec 15, 2015 at 09:52:33AM +0100, Michal Hocko wrote:
> > 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.
> 
> No, it doesn't. follow_page(FOLL_SPLIT) would split THP pages.

Ahh, I thought it would split the pmd but this path splits the page
directly.
 
> > 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?
> 
> head page is what linked into LRU, but not nessesary the way we obtain the
> page to check. If we check PageLRU(pte_page(*pte)) it should produce the
> right result.

I am not following you here. Any pfn walk could get to a tail page and
if we happen to do e.g. isolate_lru_page we have to remember that we
should always treat compound page differently. This is subtle. Anyway I
am far from understading other parts of the refcount rework so I will
spend time studying the code as soon as the time permits. In the
meantime I agree that VM_BUG_ON_PAGE(PageTail(page), page) would be
useful to catch all the fallouts.
-- 
Michal Hocko
SUSE Labs

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Re: isolate_lru_page on !head pages

[Vlastimil Babka]

On 12/15/2015 05:59 PM, Michal Hocko wrote:
>>
>> head page is what linked into LRU, but not nessesary the way we obtain the
>> page to check. If we check PageLRU(pte_page(*pte)) it should produce the
>> right result.
>
> I am not following you here. Any pfn walk could get to a tail page and
> if we happen to do e.g. isolate_lru_page we have to remember that we
> should always treat compound page differently. This is subtle.

I think the problem is that isolate_lru_page() is not the only reason 
for calling PageLRU(). And the other use cases have different 
expectations, to either way (PF_ANY or PF_HEAD) you pick for PageLRU(), 
somebody will have to be careful. IMHO usually it's pfn scanners who 
have to be careful for many reasons...

> Anyway I
> am far from understading other parts of the refcount rework so I will
> spend time studying the code as soon as the time permits. In the
> meantime I agree that VM_BUG_ON_PAGE(PageTail(page), page) would be
> useful to catch all the fallouts.

+1

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