Re: [PATCH 1/6] mlock: only hold mmap_sem in shared mode when faulting in pages

[Michel Lespinasse <walken@google.com>]

On Thu, Dec 9, 2010 at 10:11 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Wednesday, December 8, 2010, Michel Lespinasse <walken@google.com> wrote:
>>
>> Yes, patch 1/6 changes the long hold time to be in read mode instead
>> of write mode, which is only a band-aid. But, this prepares for patch
>> 5/6, which releases mmap_sem whenever there is contention on it or
>> when blocking on disk reads.
>
> I have to say that I'm not a huge fan of that horribly kludgy
> contention check case.
>
> The "move page-in to read-locked sequence" and the changes to
> get_user_pages look fine, but the contention thing is just disgusting.
> I'd really like to see some other approach if at all possible.

Andrew, should I amend my patches to remove the rwsem_is_contended() code ?
This would involve:
- remove rwsem-implement-rwsem_is_contended.patch and
  x86-rwsem-more-precise-rwsem_is_contended-implementation.patch
- in mlock-do-not-hold-mmap_sem-for-extended-periods-of-time.patch,
  drop the one hunk making use of rwsem_is_contended (rest of the patch
  would still work without it)
- optionally, follow up patch to limit batch size to a constant
  in do_mlock_pages():

diff --git a/mm/mlock.c b/mm/mlock.c
index 569ae6a..a505a7e 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -457,15 +457,23 @@ static int do_mlock_pages(unsigned long start, size_t len)
 			continue;
 		if (nstart < vma->vm_start)
 			nstart = vma->vm_start;
+		/*
+		 * Constrain batch size to limit mmap_sem hold time.
+		 */
+		if (nend > nstart + 1024 * PAGE_SIZE)
+			nend = nstart + 1024 * PAGE_SIZE;
 		/*
 		 * Now fault in a range of pages. __mlock_vma_pages_range()
 		 * double checks the vma flags, so that it won't mlock pages
 		 * if the vma was already munlocked.
 		 */
 		ret = __mlock_vma_pages_range(vma, nstart, nend, &locked);
 		if (ret < 0) {
 			ret = __mlock_posix_error_return(ret);
 			break;
+		} else if (locked) {
+			locked = 0;
+			up_read(&mm->mmap_sem);
 		}
 		nend = nstart + ret * PAGE_SIZE;
 		ret = 0;


I don't really prefer using a constant, but I'm not sure how else to make
Linus happy :)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.