Messages in this thread | | | Date | Mon, 1 Dec 2008 09:58:44 +0100 | From | Nick Piggin <> | Subject | Re: [RFC v1][PATCH]page_fault retry with NOPAGE_RETRY |
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On Sun, Nov 30, 2008 at 09:54:56PM +0200, Török Edwin wrote: > On 2008-11-29 01:02, Mike Waychison wrote: > > Nick Piggin wrote: > >> On Thu, Nov 27, 2008 at 11:03:40AM -0800, Mike Waychison wrote: > >>> Nick Piggin wrote: > >>>> On Thu, Nov 27, 2008 at 01:28:41AM -0800, Mike Waychison wrote: > >>>>> Török however identified mmap taking on the order of several > >>>>> milliseconds due to this exact problem: > >>>>> > >>>>> http://lkml.org/lkml/2008/9/12/185 > >>>> Turns out to be a different problem. > >>>> > >>> What do you mean? > >> > >> His is just contending on the write side. The retry patch doesn't help. > >> > > > > I disagree. How do you get 'write contention' from the following > > paragraph: > > > > "Just to confirm that the problem is with pagefaults and mmap, I dropped > > the mmap_sem in filemap_fault, and then > > I got same performance in my testprogram for mmap and read. Of course > > this is totally unsafe, because the mapping could change at any time." > > > > It reads to me that the writers were held off by the readers sleeping > > in IO. > > It is true that I have a write/write contention too, but do_page_fault > shows up too on lock_stat. > > This is my guess at what happens: > * filemap_fault used to sleep with mmap_sem held while waiting for the > page lock. > * the google patch avoids that, which is fine: if page lock can't be > taken, it drops mmap_sem, waits, then retries the fault once > * however after we acquired the page lock, mapping->a_ops->readpage is > invoked, mmap_sem is NOT dropped here: > > error = mapping->a_ops->readpage(file, page); > if (!error) { > wait_on_page_locked(page); > > If my understanding is correct ->readpage does the actual disk I/O, and > it keeps the page locked, when the lock is released we know it has finished. > So wait_on_page_locked(page) holds mmap_sem locked for read during the > disk I/O, preventing sys_mmap/sys_munmap from making progress.
Yes that's exactly right. Ahh, the google patch doesn't solve this case? Interesting...
> I don't know how to prove/disprove my guess above, suggestions welcome. > > Could the patch be changed to also release the mmap_sem after readpage, > and before wait_on_page_locked?
It should be possible somehow, but it is difficult because after dropping mmap_sem, then we have to basically retry the whole fault because the vma might have gone away.
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