linux-kernel - Re: [Xen-devel] [PATCH] xen/gntdev: Use mempolicy instead of VM

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Date:   Wed, 16 Nov 2016 17:20:46 +0000
From:   David Vrabel <david.vrabel@...rix.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        <david.vrabel@...rix.com>, <jgross@...e.com>
CC:     <xen-devel@...ts.xenproject.org>, <olaf@...fle.de>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen/gntdev: Use mempolicy instead of VM_IO
 flag to avoid NUMA balancing

On 16/11/16 17:02, Boris Ostrovsky wrote:
> Commit 9c17d96500f7 ("xen/gntdev: Grant maps should not be subject to
> NUMA balancing") set VM_IO flag to prevent grant maps from being
> subjected to NUMA balancing.
> 
> It was discovered recently this this flag may cause page allocation
> failures with the following stack:

It's not an allocation failure.

"It was discovered recently that this flag causes get_user_pages() to
always fail with -EFAULT."

> check_vma_flags
> __get_user_pages
> __get_user_pages_locked
> __get_user_pages_unlocked
> get_user_pages_fast
> iov_iter_get_pages
> dio_refill_pages
> do_direct_IO
> do_blockdev_direct_IO
> do_blockdev_direct_IO
> ext4_direct_IO_read
> generic_file_read_iter
> aio_run_iocb
> 
> (which can happen if guest's vdisk has direct-io-safe option).
> 
> To avoid this, instead of setting VM_IO use mempolicy that prohibits page
> migration (i.e. clear policy's MPOL_F_MOF|MPOL_F_MORON)

Reviewed-by: David Vrabel <david.vrabel@...rix.com>

With the corrected commit message.

David