Messages in this thread | | | Date | Sun, 10 Jul 2011 08:59:06 -0700 | From | Wu Fengguang <> | Subject | Re: [TOME] Re: [PATCH 3/3] mm/readahead: Move the check for ra_pages after VM_SequentialReadHint() |
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On Sun, Jul 10, 2011 at 08:59:09PM +0800, Raghavendra D Prabhu wrote: > * On Sat, Jul 09, 2011 at 01:53:08PM -0700, Wu Fengguang <fengguang.wu@intel.com> wrote: > >On Sun, Jul 10, 2011 at 03:41:20AM +0800, Raghavendra D Prabhu wrote: > >>page_cache_sync_readahead checks for ra->ra_pages again, so moving the check after VM_SequentialReadHint. > > > >NAK. This patch adds nothing but overheads. > > > >>--- a/mm/filemap.c > >>+++ b/mm/filemap.c > >>@@ -1566,8 +1566,6 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma, > >> /* If we don't want any read-ahead, don't bother */ > >> if (VM_RandomReadHint(vma)) > >> return; > >>- if (!ra->ra_pages) > >>- return; > > >> if (VM_SequentialReadHint(vma)) { > >> page_cache_sync_readahead(mapping, ra, file, offset, > >>@@ -1575,6 +1573,9 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma, > >> return; > >> } > > >>+ if (!ra->ra_pages) > >>+ return; > >>+ > > > >page_cache_sync_readahead() has the same > > > > if (!ra->ra_pages) > > return; > 1. Yes, I saw that and that is why I moved it after the condition, so that duplicate checks are > not needed -- ie., if VM_SequentialReadHint is true, then > (!ra->ra_pages) is checked twice otherwise.
Ok, I see.
> 2. Also, another thought, is the check needed at its original place (if > not it can be removed), reasons being -- filesystems like tmpfs which > have ra_pages set to 0 don't use filemap_fault in their VMA ops and also
Good point. tmpfs is using shmem_fault().. Can you remove the test?
> do_sync_mmap_readahead is called in a major page fault context.
Right. This is irrelevant however, because if pa_pages==0, the page faults will normally be major ones.
Thanks, Fengguang
> >So the patch adds the call into page_cache_sync_readahead() just to return.. > > > >Thanks, > >Fengguang > > > -------------------------- > Raghavendra Prabhu > GPG Id : 0xD72BE977 > Fingerprint: B93F EBCB 8E05 7039 CD3C A4B8 A616 DCA1 D72B E977 > www: wnohang.net
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