Re: [PATCHv3 03/24] mm: avoid PG_locked on tail pages

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

"Kirill A. Shutemov" <kirill@shutemov.name> writes:

> On Tue, Mar 03, 2015 at 06:51:11PM +0530, Aneesh Kumar K.V wrote:
>> Rik van Riel <riel@redhat.com> writes:
>> 
>> > -----BEGIN PGP SIGNED MESSAGE-----
>> > Hash: SHA1
>> >
>> > On 02/12/2015 02:55 PM, Rik van Riel wrote:
>> >> On 02/12/2015 11:18 AM, Kirill A. Shutemov wrote:
>> >
>> >>> @@ -490,6 +493,7 @@ extern int 
>> >>> wait_on_page_bit_killable_timeout(struct page *page,
>> >> 
>> >>> static inline int wait_on_page_locked_killable(struct page *page)
>> >>>  { +	page = compound_head(page); if (PageLocked(page)) return 
>> >>> wait_on_page_bit_killable(page, PG_locked); return 0; @@ -510,6 
>> >>> +514,7 @@ static inline void wake_up_page(struct page *page, int 
>> >>> bit) */ static inline void wait_on_page_locked(struct page *page)
>> >>>  { +	page = compound_head(page); if (PageLocked(page)) 
>> >>> wait_on_page_bit(page, PG_locked); }
>> >> 
>> >> These are all atomic operations.
>> >> 
>> >> This may be a stupid question with the answer lurking somewhere in
>> >> the other patches, but how do you ensure you operate on the right
>> >> page lock during a THP collapse or split?
>> >
>> > Kirill answered that question on IRC.
>> >
>> > The VM takes a refcount on a page before attempting to take a page
>> > lock, which prevents the THP code from doing anything with the
>> > page. In other words, while we have a refcount on the page, we
>> > will dereference the same page lock.
>> 
>> Can we explain this more ? Don't we allow a thp split to happen even if
>> we have page refcount ?.
>
> The patchset changes this. Have you read the cover letter?
>

Ok got that.

Thanks,
-aneesh

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