Messages in this thread | | | Date | Sun, 30 Nov 2008 21:54:56 +0200 | From | Török Edwin <> | Subject | Re: [RFC v1][PATCH]page_fault retry with NOPAGE_RETRY |
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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.
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?
Best regards, --Edwin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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