Re: [PATCHv3 29/41] ext4: make ext4_mpage_readpages() hugepage-aware

[Kirill A. Shutemov]

On Thu, Sep 15, 2016 at 06:27:10AM -0600, Andreas Dilger wrote:
> On Sep 15, 2016, at 5:55 AM, Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > 
> > This patch modifies ext4_mpage_readpages() to deal with huge pages.
> > 
> > We read out 2M at once, so we have to alloc (HPAGE_PMD_NR *
> > blocks_per_page) sector_t for that. I'm not entirely happy with kmalloc
> > in this codepath, but don't see any other option.
> 
> If you're reading 2MB from disk (possibly from disjoint blocks with seeks
> in between) I don't think that the kmalloc() is going to be the limiting
> performance factor.  If you are concerned about the size of the kmalloc()
> causing failures when pages are fragmented (it can be 16KB for 1KB blocks
> with 4KB pages), then using ext4_kvmalloc() to fall back to vmalloc() in
> case kmalloc() fails.  It shouldn't fail often for 16KB allocations,
> but it could in theory.

Good point. Will use ext4_kvmalloc().

> I also notice that ext4_kvmalloc() should probably use unlikely() for
> the failure case, so that the uncommon vmalloc() fallback is out-of-line
> in this more important codepath.  The only other callers are during mount,
> so a branch misprediction is not critical.

I agree. But it's out-of-scope for the patchset.

-- 
 Kirill A. Shutemov