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

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> 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.


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 :)




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