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

[Vlastimil Babka <vbabka@suse.cz>]

On 2/25/19 5:36 AM, 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
> reused and that pages allocated to the frag pool are pinned in
> memory until all fragments allocated on the page have been freed?

I don't know if you did, but it's certainly true., Also I don't think
there's any specified alignment guarantee for page_frag_alloc().

What about kmem_cache_create() with align parameter? That *should* be
guaranteed regardless of whatever debugging is enabled - if not, I would
consider it a bug.

> 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...
> 
> Cheers,
> 
> Dave.
>