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    SubjectRe: [PATCH] dm: max_segments=1 if merge_bvec_fn is not supported
    On Thu, 18 Mar 2010 11:48:58 -0700
    Andrew Morton <akpm@linux-foundation.org> wrote:

    > On Mon, 8 Mar 2010 14:14:49 +0100
    > Lars Ellenberg <lars.ellenberg@linbit.com> wrote:
    >
    > > On Mon, Mar 08, 2010 at 03:35:37AM -0500, Mikulas Patocka wrote:
    > > > Hi
    > > >
    > > > That patch with limits->max_segments = 1; is wrong. It fixes this bug
    > > > sometimes and sometimes not.
    > > >
    > > > The problem is, if someone attempts to create a bio with two vector
    > > > entries, the first maps the last sector contained in some page and the
    > > > second maps the first sector of the next physical page: it has one
    > > > segment, it has size <= PAGE_SIZE, but it still may cross raid stripe and
    > > > the raid driver will reject it.
    > >
    > > Now that you put it that way ;)
    > > You are right.
    > >
    > > My asumption that "single segment" was
    > > equalvalent in practice with "single bvec"
    > > does not hold true in that case.
    > >
    > > Then, what about adding seg_boundary_mask restrictions as well?
    > > max_sectors = PAGE_SIZE >> 9;
    > > max_segments = 1;
    > > seg_boundary_mask = PAGE_SIZE -1;
    > > or some such.
    > >
    > > > > > This is not the first time this has been patched, btw.
    > > > > > See https://bugzilla.redhat.com/show_bug.cgi?id=440093
    > > > > > and the patch by Mikulas:
    > > > > > https://bugzilla.redhat.com/attachment.cgi?id=342638&action=diff
    > > >
    > > > Look at this patch, it is the proper way how to fix it: create a
    > > > merge_bvec_fn that reject more than one biovec entry.
    > >
    > > If adding seg_boundary_mask is still not sufficient,
    > > lets merge that patch instead?
    > > Why has it been dropped, respectively never been merged?
    > > It became obsolete for dm-linear by 7bc3447b,
    > > but in general the bug is still there, or am I missing something?
    > >
    >
    > This all seemed to die. Does Neil's mysterypatch fix all these issues?
    >
    > Neil, was that patch tagged for -stable backporting?

    The patch at the top of my 'for-linus' branch (which Linus doesn't seem to
    have pulled yet) fixes this for md and is tagged for -stable backporting.
    I just sets max_segments and seg_boundary_mask. There is no point setting
    max_sectors as well. I found that setting merge_bvec_fn, while a perfectly
    correct approach, was more cumbersome.

    My patch doesn't fix this for dm. I assume the dm developers will do that.

    NeilBrown


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