Re: [git patches] xfs and block fixes for virtually indexed arches

From: tytso
Date: Thu Dec 17 2009 - 12:33:59 EST

On Thu, Dec 17, 2009 at 12:10:25PM -0500, Christoph Hellwig wrote:
> On Thu, Dec 17, 2009 at 11:30:36AM -0500, tytso@xxxxxxx wrote:
> > That's because apparently the iSCSI and DMA blocks assume that they
> > have Real Pages (tm) passed to block I/O requests, and apparently XFS
> > ran into problems when sending vmalloc'ed pages. I don't know if this
> > is a problem if we pass the bio layer addresses coming from the SLAB
> > allocator, but oral tradition seems to indicate this is problematic,
> > although no one has given me the full chapter and verse explanation
> > about why this is so.
> Actually at least iscsi now has a workaround for that by checking for
> PageSlab. Back when we deal with the XFS issue that check was only
> available with debug options enabled. I tried to sort it out by
> agreeing with the block and iscsi folks that either
> a) we need to send down refcountable pages
> b) block drivers need to accept kmalloced pages
> I could not get any afreement, and thus we stopped using the kmalloced
> pages in XFS which was easy enough. A bit later people fixed iscsi,
> but we still don't have formal rules about what is acceptable to the
> block layer.

It would be good to get some formal rules articulated. Someone has
asserted that the AoE (ATA over Ethernet) driver will barf on
SLAB/kmalloc allocated memory. True, false?

- Ted
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