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    SubjectRe: [bug] block subsystem related crash with latest -git
    On Wed, Oct 17 2007, Jens Axboe wrote:
    > On Wed, Oct 17 2007, Jens Axboe wrote:
    > > On Wed, Oct 17 2007, Jens Axboe wrote:
    > > > On Wed, Oct 17 2007, Linus Torvalds wrote:
    > > > >
    > > > >
    > > > > On Wed, 17 Oct 2007, Ingo Molnar wrote:
    > > > > >
    > > > > > Jens, just got this crash on a testbox:
    > > > >
    > > > > The code in question is:
    > > > >
    > > > > mov %edx,0xc(%esp)
    > > > > mov (%ebx),%edi
    > > > > mov %edi,%edx
    > > > > sub %eax,%edx
    > > > > mov %edx,%eax
    > > > > sar $0x5,%eax
    > > > > shl $0xc,%eax
    > > > > add 0x8(%ebx),%eax
    > > > > cmp %eax,0xc(%esp)
    > > > > je +126
    > > > > mov 0x10(%esi),%eax <----- Oops
    > > > > lea 0x10(%esi),%edx
    > > > > test $0x1,%al
    > > > > jne +76
    > > > > mov %edi,(%esi)
    > > > > mov %ebp,0xc(%esi)
    > > > > mov 0x8(%ebx),%eax
    > > > > mov %eax,0x4(%esi)
    > > > >
    > > > >
    > > > > and it looks like %esi is overflowing from one page to the next one, ie:
    > > > >
    > > > > BUG: unable to handle kernel paging request at virtual address 7ca76000
    > > > > ESI: 7ca75ff0
    > > > >
    > > > > and you caught this thanks to page-alloc debugging again.
    > > > >
    > > > > I think I can match that up with the source code: that's "sg_next()". It's
    > > > > doing:
    > > > >
    > > > > sg++;
    > > > >
    > > > > if (unlikely(sg_is_chain(sg)))
    > > > > sg = sg_chain_ptr(sg);
    > > > >
    > > > > return sg;
    > > > >
    > > > > and the oopsing instruction is that load of "sg->page" in the assembly
    > > > > code:
    > > > >
    > > > > mov 0x10(%esi),%eax # %eax = sg->page
    > > > > lea 0x10(%esi),%edx # %edx = sg+1;
    > > > > test $0x1,%al # if (unlikely(sg_is_chain()))
    > > > > jne +76
    > > > >
    > > > > Jens?
    > > >
    > > > Yep, that's what I came up with as well - I asked Ingo for a dump in
    > > > private, but ended up just using ksymoops to decode the line.
    > > >
    > > > The way blk_rq_map_sg() operates is that it ends up doing a
    > > >
    > > > next_sg = sg_next(sg);
    > > >
    > > > even though sg may be the last entry. Perhaps this is crapping out,
    > > > although if sg is a valid address, then sg + 1 should be as well.
    > > > next_sg may end up being crap, in fact it will, but we'll never use that
    > > > unless there are more entries to fill. And if there is, then both sg and
    > > > next_sg were valid.
    > > >
    > > > So nothing in for-linus should fix it, I'll try and come up with an
    > > > alternate way to assign next_sg so it's always valid.
    > >
    > > OK, the below should actually be safe, I don't know why I talked myself
    > > into the next_sg stuff in the beginning. It's always safe to zero sg,
    > > since it's a valid entry - nothing to save in ->page. Ingo, does this
    > > work for you?
    > >
    > > diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
    > > index 9e3f3cc..3935469 100644
    > > --- a/block/ll_rw_blk.c
    > > +++ b/block/ll_rw_blk.c
    > > @@ -1322,8 +1322,8 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
    > > struct scatterlist *sglist)
    > > {
    > > struct bio_vec *bvec, *bvprv;
    > > - struct scatterlist *next_sg, *sg;
    > > struct req_iterator iter;
    > > + struct scatterlist *sg;
    > > int nsegs, cluster;
    > >
    > > nsegs = 0;
    > > @@ -1333,7 +1333,7 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
    > > * for each bio in rq
    > > */
    > > bvprv = NULL;
    > > - sg = next_sg = &sglist[0];
    > > + sg = NULL;
    > > rq_for_each_segment(bvec, rq, iter) {
    > > int nbytes = bvec->bv_len;
    > >
    > > @@ -1349,8 +1349,10 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
    > > sg->length += nbytes;
    > > } else {
    > > new_segment:
    > > - sg = next_sg;
    > > - next_sg = sg_next(sg);
    > > + if (!sg)
    > > + sg = sglist;
    > > + else
    > > + sg = sg_next(sg);
    > >
    > > memset(sg, 0, sizeof(*sg));
    > > sg->page = bvec->bv_page;
    > >
    >
    > Scratch that, it cannot work... I'll think up a different approach.

    OK, it is fine, as long as the sglist is cleared initially. And I don't
    think there's anyway around that, clearly I didn't think long enough
    before including the memset() removal from Tomo.

    Ingo, please try this rolled up version.

    Linus, this should work. It would probably be best if you first did a
    git revert on f5c0dde4c66421a3a2d7d6fa604a712c9b0744e5 and then applied
    the ll_rw_blk.c bit alone. Do you want me to stuff that (revert + patch)
    into a branch for you to pull?

    diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
    index 9e3f3cc..3935469 100644
    --- a/block/ll_rw_blk.c
    +++ b/block/ll_rw_blk.c
    @@ -1322,8 +1322,8 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
    struct scatterlist *sglist)
    {
    struct bio_vec *bvec, *bvprv;
    - struct scatterlist *next_sg, *sg;
    struct req_iterator iter;
    + struct scatterlist *sg;
    int nsegs, cluster;

    nsegs = 0;
    @@ -1333,7 +1333,7 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
    * for each bio in rq
    */
    bvprv = NULL;
    - sg = next_sg = &sglist[0];
    + sg = NULL;
    rq_for_each_segment(bvec, rq, iter) {
    int nbytes = bvec->bv_len;

    @@ -1349,8 +1349,10 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
    sg->length += nbytes;
    } else {
    new_segment:
    - sg = next_sg;
    - next_sg = sg_next(sg);
    + if (!sg)
    + sg = sglist;
    + else
    + sg = sg_next(sg);

    memset(sg, 0, sizeof(*sg));
    sg->page = bvec->bv_page;
    diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
    index 0c86be7..aac8a02 100644
    --- a/drivers/scsi/scsi_lib.c
    +++ b/drivers/scsi/scsi_lib.c
    @@ -764,6 +764,8 @@ struct scatterlist *scsi_alloc_sgtable(struct scsi_cmnd *cmd, gfp_t gfp_mask)
    if (unlikely(!sgl))
    goto enomem;

    + memset(sgl, 0, sizeof(*sgl) * sgp->size);
    +
    /*
    * first loop through, set initial index and return value
    */
    --
    Jens Axboe

    -
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