Re: [TOME] Re: [PATCH 3/3] mm/readahead: Move the check forra_pages after VM_SequentialReadHint()

[Wu Fengguang]

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@xxxxxxxxx> 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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