Messages in this thread | | | Date | Sun, 23 Nov 2008 10:18:44 +0100 | From | Ingo Molnar <> | Subject | Re: [RFC v1][PATCH]page_fault retry with NOPAGE_RETRY |
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* Ying Han <yinghan@google.com> wrote:
> page fault retry with NOPAGE_RETRY
Interesting patch.
> Allow major faults to drop the mmap_sem read lock while waitting for > synchronous disk read. This allows another thread which wishes to grab > down_read(mmap_sem) to proceed while the current is waitting the disk IO.
Do you mean down_write()? down_read() can already be nested arbitrarily.
> The patch flags current->flags to PF_FAULT_MAYRETRY as identify that > the caller can tolerate the retry in the filemap_fault call patch. > > Benchmark is done by mmap in huge file and spaw 64 thread each > faulting in pages in reverse order, the the result shows 8% > porformance hit with the patch.
I suspect we also want to see the cases where this change helps?
Also, constructs like this are pretty ugly:
> +#ifdef CONFIG_X86_64 > +asmlinkage > +#endif > +void do_page_fault(struct pt_regs *regs, unsigned long error_code) > +{ > + current->flags |= PF_FAULT_MAYRETRY; > + __do_page_fault(regs, error_code); > + current->flags &= ~PF_FAULT_MAYRETRY; > +}
This seems to be unnecessary runtime overhead to pass in a flag to handle_mm_fault(). Why not extend the 'write' flag of handle_mm_fault() to also signal "arch is able to retry"?
Also, _if_ we decide that from-scratch pagefault retries are good, i see no reason why this should not be extended to all architectures:
The retry should happen purely in the MM layer - all information is available already, and much of do_page_fault() could generally be moved into mm/memory.c, with one or two arch-provided standard callbacks to express certain page fault quirks. (such as vm86 mode on x86)
(Such a design would allow more nice cleanups - handle_mm_fault() could inline inside the pagefault handler, etc.)
Also, a few small details. Please use this proper multi-line comment style:
> + /* > + * Page is already locked by someone else. > + * > + * We don't want to be holding down_read(mmap_sem) > + * inside lock_page(). We use wait_on_page_lock here > + * to just wait until the page is unlocked, but we > + * don't really need > + * to lock it. > + */
Not this one:
> + /* page may be available, but we have to restart the process > + * because mmap_sem was dropped during the ->fault */
Ingo
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