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    SubjectRe: [PATCH 1/2] block: fix leaks associated with discard request payload
    On Tue, Jun 29 2010 at  7:03pm -0400,
    James Bottomley <James.Bottomley@suse.de> wrote:

    > On Tue, 2010-06-29 at 18:28 -0400, Mikulas Patocka wrote:
    > >
    > > On Sun, 27 Jun 2010, James Bottomley wrote:
    > >
    > > > linux-scsi cc added, since it's a SCSI patch.
    > > >
    > > > On Sat, 2010-06-26 at 15:56 -0400, Mike Snitzer wrote:
    > > > > Fix leaks introduced via "block: don't allocate a payload for discard
    > > > > request" commit a1d949f5f44.
    > > > >
    > > > > sd_done() is not called for REQ_TYPE_BLOCK_PC commands so cleanup
    > > > > discard request's payload directly in scsi_finish_command().
    > > > >
    > > > > Also cleanup page allocated for discard payload in
    > > > > scsi_setup_discard_cmnd's scsi_setup_blk_pc_cmnd error path.
    > > > >
    > > > > Signed-off-by: Mike Snitzer <snitzer@redhat.com>
    > > > > ---
    > > > > block/blk-core.c | 23 +++++++++++++++++++++++
    > > > > drivers/scsi/scsi.c | 8 ++++++++
    > > > > drivers/scsi/sd.c | 18 ++++++++----------
    > > > > include/linux/blkdev.h | 1 +
    > > > > 4 files changed, 40 insertions(+), 10 deletions(-)
    > > > >
    > > > > diff --git a/block/blk-core.c b/block/blk-core.c
    > > > > index 98b4cee..07925aa 100644
    > > > > --- a/block/blk-core.c
    > > > > +++ b/block/blk-core.c
    > > > > @@ -1167,6 +1167,29 @@ void blk_add_request_payload(struct request *rq, struct page *page,
    > > > > }
    > > > > EXPORT_SYMBOL_GPL(blk_add_request_payload);
    > > > >
    > > > > +/**
    > > > > + * blk_clear_request_payload - clear a request's payload
    > > > > + * @rq: request to update
    > > > > + *
    > > > > + * The driver needs to take care of freeing the payload itself.
    > > > > + */
    > > > > +void blk_clear_request_payload(struct request *rq)
    > > > > +{
    > > > > + struct bio *bio = rq->bio;
    > > > > +
    > > > > + rq->__data_len = rq->resid_len = 0;
    > > > > + rq->nr_phys_segments = 0;
    > > > > + rq->buffer = NULL;
    > > > > +
    > > > > + bio->bi_size = 0;
    > > > > + bio->bi_vcnt = 0;
    > > > > + bio->bi_phys_segments = 0;
    > > > > +
    > > > > + bio->bi_io_vec->bv_page = NULL;
    > > > > + bio->bi_io_vec->bv_len = 0;
    > > > > +}
    > > > > +EXPORT_SYMBOL_GPL(blk_clear_request_payload);
    > > > > +
    > > > > void init_request_from_bio(struct request *req, struct bio *bio)
    > > > > {
    > > > > req->cpu = bio->bi_comp_cpu;
    > > > > diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
    > > > > index ad0ed21..69c7ea4 100644
    > > > > --- a/drivers/scsi/scsi.c
    > > > > +++ b/drivers/scsi/scsi.c
    > > > > @@ -851,6 +851,14 @@ void scsi_finish_command(struct scsi_cmnd *cmd)
    > > > > */
    > > > > if (good_bytes == old_good_bytes)
    > > > > good_bytes -= scsi_get_resid(cmd);
    > > > > + } else if (cmd->request->cmd_flags & REQ_DISCARD) {
    > > > > + /*
    > > > > + * If this is a discard request that originated from the kernel
    > > > > + * we need to free our payload here. Note that we need to check
    > > > > + * the request flag as the normal payload rules apply for
    > > > > + * pass-through UNMAP / WRITE SAME requests.
    > > > > + */
    > > > > + __free_page(bio_page(cmd->request->bio));
    > > >
    > > > This is another layering violation: the page is allocated in the Upper
    > > > layer and freed in the mid-layer.
    > > >
    > > > I really hate these growing contortions for discard. They're a clear
    > > > signal that we haven't implemented it right.
    > > >
    > > > So let's first work out how it should be done. I really like Tomo's
    > > > idea of doing discard through the normal REQ_TYPE_FS route, which means
    > > > we can control the setup in prep and the tear down in done, all confined
    > > > to the ULD.
    > > >
    > > > Where I think I'm at is partially what Christoph says: The command
    > > > transformation belongs in the ULD so that's where the allocation and
    > > > deallocation should be, and partly what Tomo says in that we should
    > > > eliminate the special case paths.
    > > >
    > > > The payload vs actual request size should be a red herring if we've got
    > > > everything correct: only the ULD cares about the request parameters.
    > > > Once we've got everything set up, the mid layer and LLD should only care
    > > > about the parameters in the command, so we can confine the size changing
    > > > part to the ULD doing the discard.
    > > >
    > > > Could someone take a stab at this and see if it works without layering
    > > > violations or any other problematic signals?
    > > >
    > > > Thanks,
    > > >
    > > > James
    > >
    > > Well, I think that you overestimate the importance of scsi code too much.
    >
    > Not, I think, a deadly sin for a SCSI maintainer.

    Indeed ;)

    > > There is a layering violation in the code. So what --- you either fix the
    > > layering violation or let it be there and grind your teeth on it. But in
    > > either case, that layering violation won't affect anyone except scsi
    > > developers.
    >
    > A layering violation is a signal of bad design wherever it occurs, so
    > that wasn't a SCSI centric argument.
    >
    > > On the other hand, if you say "because we want to avoid layering violation
    > > in SCSI, every issuer of discard request must supply an empty page", you
    > > create havoc all over the Linux codebase. md, dm, drbd, xvd, virtio ---
    > > whatever you think of, will be allocating a dummy page when constructing
    > > a discard request.
    >
    > Since I didn't actually say any of that, I suggest you re-read text you
    > quoted above. The phrase "The command transformation belongs in the ULD
    > so that's where the allocation and deallocation should be" might be a
    > relevant one to concentrate on.

    Right, freeing the page, that was allocated in SCSI's ULD, from the SCSI
    midlayer is a SCSI layering violation. I think Mikulas was reacting to
    the desire to maintain the existing, arguably more problematic, layering
    violation that spans the block and SCSI layers.

    > > If the layering violation spans only scsi code, it can be eventually
    > > fixed, but this, much worse "layering violation" that will be spanning all
    > > block device midlayers, won't ever be fixed.
    > >
    > > Imagine for example --- a discard request arrivers at a dm-snapshot
    > > device. The driver splits it into chunks, remaps each chunk to the
    > > physical chunk, submits the requests, the elevator merges adjacent
    > > requests and submits fewer bigger requests to the device. Now, if you had
    > > to allocate a zeroed page each time you are splitting the request, that
    > > would exhaust memory and burn cpu needlessly. You delete a 100MB file? ---
    > > fine, allocate a 100MB of zeroed pages.
    >
    > This is a straw man: You've tried to portray a position I've never
    > taken as mine then attack it ... with what is effectively another bogus
    > argument.
    >
    > It's not an either/or choice. I've asked the relevant parties to
    > combine the approaches and see if a REQ_TYPE_FS path that does the
    > allocations in the appropriate place, likely the ULD, produces a good
    > design.

    If in the end we can fix up SCSI properly then everyone is happy. So
    lets just keep working toward that. The various attempts to convert
    discard over to REQ_TYPE_FS have fallen short. Hopefully we'll have a
    break through shortly.

    Thanks for your guidance James,
    Mike


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