Re: [PATCH] btrfs: fix allocation of bitmap pages.

On 8/20/19 4:30 AM, Christoph Hellwig wrote:

On Mon, Aug 19, 2019 at 07:46:00PM +0200, David Sterba wrote:

quoted

Another thing that is lost is the slub debugging support for all
architectures, because get_zeroed_pages lacking the red zones and sanity
checks.

I find working with raw pages in this code a bit inconsistent with the
rest of btrfs code, but that's rather minor compared to the above.

Summing it up, I think that the proper fix should go to copy_page
implementation on architectures that require it or make it clear what
are the copy_page constraints.

The whole point of copy_page is to copy exactly one page and it makes
sense to assume that is aligned.  A sane memcpy would use the same
underlying primitives as well after checking they fit.  So I think the
prime issue here is btrfs' use of copy_page instead of memcpy.  The
secondary issue is slub fucking up alignments for no good reason.  We
just got bitten by that crap again in XFS as well :(

Meh, I should finally get back to https://lwn.net/Articles/787740/ right