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

Ming Lei
2019-02-25T08:48:23Z

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.

Thanks,
Ming