Re: [PATCH] xfs: allocate sector sized IO buffer via page_frag_alloc

[Ming Lei <ming.lei@redhat.com>]

On Mon, Feb 25, 2019 at 04:46:25PM +0800, Ming Lei wrote:
> On Mon, Feb 25, 2019 at 03:36:48PM +1100, Dave Chinner wrote:
> > On Mon, Feb 25, 2019 at 12:09:04PM +0800, Ming Lei wrote:
> > > XFS uses kmalloc() to allocate sector sized IO buffer.
> > ....
> > > Use page_frag_alloc() to allocate the sector sized buffer, then the
> > > above issue can be fixed because offset_in_page of allocated buffer
> > > is always sector aligned.
> > 
> > Didn't we already reject this approach because page frags cannot be
> 
> I remembered there is this kind of issue mentioned, but just not found
> the details, so post out the patch for restarting the discussion.
> 
> > reused and that pages allocated to the frag pool are pinned in
> > memory until all fragments allocated on the page have been freed?
> 
> Yes, that is one problem. But if one page is consumed, sooner or later,
> all fragments will be freed, then the page becomes available again.
> 
> > 
> > i.e. when we consider 64k page machines and 4k block sizes (i.e.
> > default config), every single metadata allocation is a sub-page
> > allocation and so will use this new page frag mechanism. IOWs, it
> > will result in fragmenting memory severely and typical memory
> > reclaim not being able to fix it because the metadata that pins each
> > page is largely unreclaimable...
> 
> It can be an issue in case of IO timeout & retry.

The worst case is still not worse than allocating single page for sub-page
IO, which should be used on other file systems under the same situation,
I guess.

thanks,
Ming