* Question on follow_page_mask
@ 2016-02-23 13:15 Anshuman Khandual
2016-02-23 14:03 ` Kirill A. Shutemov
0 siblings, 1 reply; 5+ messages in thread
From: Anshuman Khandual @ 2016-02-23 13:15 UTC (permalink / raw)
To: Kirill A. Shutemov, kirill.shutemov, Hugh Dickins
Cc: Linux PPC dev, Aneesh Kumar K.V
Not able to understand the first code block of follow_page_mask
function. follow_huge_addr function is expected to find the page
struct for the given address if it turns out to be a HugeTLB page
but then when it finds the page we bug on if it had been called
with FOLL_GET flag.
page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
if (!IS_ERR(page)) {
BUG_ON(flags & FOLL_GET);
return page;
}
do_move_page_to_node_array calls follow_page with FOLL_GET which
in turn calls follow_page_mask with FOLL_GET. On POWER, the
function follow_huge_addr is defined and does not return -EINVAL
like the generic one. It returns the page struct if its a HugeTLB
page. Just curious to know what is the purpose behind the BUG_ON.
Thank you.
Regards
Anshuman
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Question on follow_page_mask
2016-02-23 13:15 Question on follow_page_mask Anshuman Khandual
@ 2016-02-23 14:03 ` Kirill A. Shutemov
2016-02-23 21:07 ` Hugh Dickins
2016-02-24 11:22 ` Anshuman Khandual
0 siblings, 2 replies; 5+ messages in thread
From: Kirill A. Shutemov @ 2016-02-23 14:03 UTC (permalink / raw)
To: Anshuman Khandual
Cc: kirill.shutemov, Hugh Dickins, Linux PPC dev, Aneesh Kumar K.V
On Tue, Feb 23, 2016 at 06:45:05PM +0530, Anshuman Khandual wrote:
> Not able to understand the first code block of follow_page_mask
> function. follow_huge_addr function is expected to find the page
> struct for the given address if it turns out to be a HugeTLB page
> but then when it finds the page we bug on if it had been called
> with FOLL_GET flag.
>
> page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
> if (!IS_ERR(page)) {
> BUG_ON(flags & FOLL_GET);
> return page;
> }
>
> do_move_page_to_node_array calls follow_page with FOLL_GET which
> in turn calls follow_page_mask with FOLL_GET. On POWER, the
> function follow_huge_addr is defined and does not return -EINVAL
> like the generic one. It returns the page struct if its a HugeTLB
> page. Just curious to know what is the purpose behind the BUG_ON.
I would guess requesting pin on non-reclaimable page is considered
useless, meaning suspicius behavior. BUG_ON() is overkill, I think.
WARN_ON_ONCE() would make it.
Not that this follow_huge_addr() on Power is not reachable via
do_move_page_to_node_array(), because the vma is !vma_is_migratable().
--
Kirill A. Shutemov
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Question on follow_page_mask
2016-02-23 14:03 ` Kirill A. Shutemov
@ 2016-02-23 21:07 ` Hugh Dickins
2016-02-24 11:45 ` Anshuman Khandual
2016-02-24 11:22 ` Anshuman Khandual
1 sibling, 1 reply; 5+ messages in thread
From: Hugh Dickins @ 2016-02-23 21:07 UTC (permalink / raw)
To: Kirill A. Shutemov
Cc: Anshuman Khandual, kirill.shutemov, Hugh Dickins, Linux PPC dev,
Aneesh Kumar K.V, Naoya Horiguchi
On Tue, 23 Feb 2016, Kirill A. Shutemov wrote:
> On Tue, Feb 23, 2016 at 06:45:05PM +0530, Anshuman Khandual wrote:
> > Not able to understand the first code block of follow_page_mask
> > function. follow_huge_addr function is expected to find the page
> > struct for the given address if it turns out to be a HugeTLB page
> > but then when it finds the page we bug on if it had been called
> > with FOLL_GET flag.
> >
> > page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
> > if (!IS_ERR(page)) {
> > BUG_ON(flags & FOLL_GET);
> > return page;
> > }
> >
> > do_move_page_to_node_array calls follow_page with FOLL_GET which
> > in turn calls follow_page_mask with FOLL_GET. On POWER, the
> > function follow_huge_addr is defined and does not return -EINVAL
> > like the generic one. It returns the page struct if its a HugeTLB
> > page. Just curious to know what is the purpose behind the BUG_ON.
>
> I would guess requesting pin on non-reclaimable page is considered
> useless, meaning suspicius behavior. BUG_ON() is overkill, I think.
> WARN_ON_ONCE() would make it.
No, it's there to guard against abuse, until the correct functionality
is implemented: which has not so far been required, I think.
The problem is that a get_page() here is too late: it needs to be done
inside each arch's implementation of follow_huge_addr(), while holding
whatever is the appropriate lock, dropped by the time it returns here.
If you look through where FOLL_GET is usually implemented, such as in
follow_page_pte(), but pud and pmd cases too, I hope you'll still find
that they are careful to get the reference on the page while it's safe
in the pagetable.
But follow_huge_addr() would need some work to offer the same guarantees:
it's good for those "peep at a page without actually getting a reference"
cases, but not good enough for preventing a page for being put to some
other use completely, before we've secured it with our reference.
Unless something's changed: the last time I recall the issue coming up,
was when Naoya Horiguchi was working on hugetlbfs page migration: see
linux-kernel/linux-mm mail thread "BUG at mm/memory.c:1489!" from
28 May 2014; and the resolution there was not to support the
follow_huge_addr() case (which IIRC is peculiar to powerpc alone?).
Hugh
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Question on follow_page_mask
2016-02-23 14:03 ` Kirill A. Shutemov
2016-02-23 21:07 ` Hugh Dickins
@ 2016-02-24 11:22 ` Anshuman Khandual
1 sibling, 0 replies; 5+ messages in thread
From: Anshuman Khandual @ 2016-02-24 11:22 UTC (permalink / raw)
To: Kirill A. Shutemov
Cc: Linux PPC dev, Hugh Dickins, Aneesh Kumar K.V, kirill.shutemov
On 02/23/2016 07:33 PM, Kirill A. Shutemov wrote:
> On Tue, Feb 23, 2016 at 06:45:05PM +0530, Anshuman Khandual wrote:
>> Not able to understand the first code block of follow_page_mask
>> function. follow_huge_addr function is expected to find the page
>> struct for the given address if it turns out to be a HugeTLB page
>> but then when it finds the page we bug on if it had been called
>> with FOLL_GET flag.
>>
>> page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
>> if (!IS_ERR(page)) {
>> BUG_ON(flags & FOLL_GET);
>> return page;
>> }
>>
>> do_move_page_to_node_array calls follow_page with FOLL_GET which
>> in turn calls follow_page_mask with FOLL_GET. On POWER, the
>> function follow_huge_addr is defined and does not return -EINVAL
>> like the generic one. It returns the page struct if its a HugeTLB
>> page. Just curious to know what is the purpose behind the BUG_ON.
>
> I would guess requesting pin on non-reclaimable page is considered
> useless, meaning suspicius behavior. BUG_ON() is overkill, I think.
> WARN_ON_ONCE() would make it.
pin on non-reclaimable page ? I thought the page reference is obtained
for page migration purpose only. I may be missing something here.
>
> Not that this follow_huge_addr() on Power is not reachable via
> do_move_page_to_node_array(), because the vma is !vma_is_migratable().
Was experimenting with that enabled via ARCH_ENABLE_HUGEPAGE_MIGRATION.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Question on follow_page_mask
2016-02-23 21:07 ` Hugh Dickins
@ 2016-02-24 11:45 ` Anshuman Khandual
0 siblings, 0 replies; 5+ messages in thread
From: Anshuman Khandual @ 2016-02-24 11:45 UTC (permalink / raw)
To: Hugh Dickins, Kirill A. Shutemov
Cc: Linux PPC dev, Aneesh Kumar K.V, Naoya Horiguchi, kirill.shutemov
On 02/24/2016 02:37 AM, Hugh Dickins via Linuxppc-dev wrote:
> On Tue, 23 Feb 2016, Kirill A. Shutemov wrote:
>> On Tue, Feb 23, 2016 at 06:45:05PM +0530, Anshuman Khandual wrote:
>>> Not able to understand the first code block of follow_page_mask
>>> function. follow_huge_addr function is expected to find the page
>>> struct for the given address if it turns out to be a HugeTLB page
>>> but then when it finds the page we bug on if it had been called
>>> with FOLL_GET flag.
>>>
>>> page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
>>> if (!IS_ERR(page)) {
>>> BUG_ON(flags & FOLL_GET);
>>> return page;
>>> }
>>>
>>> do_move_page_to_node_array calls follow_page with FOLL_GET which
>>> in turn calls follow_page_mask with FOLL_GET. On POWER, the
>>> function follow_huge_addr is defined and does not return -EINVAL
>>> like the generic one. It returns the page struct if its a HugeTLB
>>> page. Just curious to know what is the purpose behind the BUG_ON.
>>
>> I would guess requesting pin on non-reclaimable page is considered
>> useless, meaning suspicius behavior. BUG_ON() is overkill, I think.
>> WARN_ON_ONCE() would make it.
Hugh, thanks for such a detailed response.
>
> No, it's there to guard against abuse, until the correct functionality
> is implemented: which has not so far been required, I think.
>
> The problem is that a get_page() here is too late: it needs to be done
> inside each arch's implementation of follow_huge_addr(), while holding
> whatever is the appropriate lock, dropped by the time it returns here.
Got it.
>
> If you look through where FOLL_GET is usually implemented, such as in
> follow_page_pte(), but pud and pmd cases too, I hope you'll still find
> that they are careful to get the reference on the page while it's safe
> in the pagetable.
yeah, true.
>
> But follow_huge_addr() would need some work to offer the same guarantees:
> it's good for those "peep at a page without actually getting a reference"
> cases, but not good enough for preventing a page for being put to some
> other use completely, before we've secured it with our reference.
Yeah, I understand that now.
>
> Unless something's changed: the last time I recall the issue coming up,
> was when Naoya Horiguchi was working on hugetlbfs page migration: see
> linux-kernel/linux-mm mail thread "BUG at mm/memory.c:1489!" from
> 28 May 2014; and the resolution there was not to support the
> follow_huge_addr() case (which IIRC is peculiar to powerpc alone?).
Yeah correct, looked into that discussion. Also tried out the discussed
small patch where we do a get_page(page) if the returned page is a head
of a HugeTLB compound page and the flag contains FOLL_GET.
That made the do_move_page_to_node_array function work before hitting a
race condition with the test case in the commit message of e66f17ff7177
("mm/hugetlb: take page table lock in follow_huge_pmd()").
I believe the test exposes the locking problem which is not being taken
care at the follow_huge_addr function level right now on powerpc and
forces it to call follow_huge_pmd (which does a BUG_ON() on powerpc)
after failing to detect a huge page at the PMD level when it checked
the first time around.
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2016-02-24 11:46 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2016-02-23 13:15 Question on follow_page_mask Anshuman Khandual
2016-02-23 14:03 ` Kirill A. Shutemov
2016-02-23 21:07 ` Hugh Dickins
2016-02-24 11:45 ` Anshuman Khandual
2016-02-24 11:22 ` Anshuman Khandual
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