Re: [PATCH] mmotm: ignore sigkill in get_user_pages during munlock

[Ying Han <yinghan@google.com>]

On Wed, Dec 3, 2008 at 4:30 PM, KOSAKI Motohiro
<kosaki.motohiro@jp.fujitsu.com> wrote:
>> PATCH ignore sigkill in get_user_pages during munlock
>>
>> Against:  2.6.28-rc7-mmotm-081203-0150
>>
>> Fixes:  make-get_user_pages-interruptible.patch
>>
>> An unfortunate side effect of "make-get_user_pages-interruptible"
>> is that it prevents a SIGKILL'd task from munlock-ing pages that it
>> had mlocked, resulting in freeing of mlocked pages.  Freeing of mlocked
>> pages, in itself, is not so bad.  We just count them now--altho' I
>> had hoped to remove this stat and add PG_MLOCKED to the free pages
>> flags check.
>>
>> However, consider pages in shared libraries mapped by more than one
>> task that a task mlocked--e.g., via mlockall().  If the task that
>> mlocked the pages exits via SIGKILL, these pages would be left mlocked
>> and unevictable.
>
> Indeed!
> Thank your for clarification!
>
> Ying, I'd like to explain unevictable lru design for you a bit more.
>
> __get_user_pages() also called exit(2) path.
>
> do_exit()
>  exit_mm()
>    mmput()
>      exit_mmap()
>        munlock_vma_pages_all()
>          munlock_vma_pages_range()
>            __mlock_vma_pages_range()
>              __get_user_pages()
>
> __mlock_vma_pages_range() process
>  (1) grab mlock related pages by __get_user_pages()
>  (2) isolate the page from lru
>  (3) the page move to evictable list if possible
>
> if (1) is interupptible, the page left unevictable lru
> although the page is not mlocked already.

Thanks KOSAKI, that is clear now. and thanks Lee for the patch. :-)

--Ying
>
> this feature was introduced 2.6.28-rc1. So I should noticed
> at last review, very sorry.
>
>
> this patch is definitly needed.
>
>        Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
>
>>
>> Proposed fix:
>>
>> Add another GUP flag to ignore sigkill when calling get_user_pages
>> from munlock()--similar to Kosaki Motohiro's 'IGNORE_VMA_PERMISSIONS
>> flag for the same purpose.  We are not actually allocating memory in
>> this case, which "make-get_user_pages-interruptible" intends to avoid.
>> We're just munlocking pages that are already resident and mapped, and
>> we're reusing get_user_pages() to access those pages.
>>
>> ?? Maybe we should combine 'IGNORE_VMA_PERMISSIONS and '_IGNORE_SIGKILL
>> into a single flag:  GUP_FLAGS_MUNLOCK ???
>
> In my personal feeling, I like current two flags :)
>
>
>
>
>

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>