Re: [PATCH v4 1/6] mm/zswap: change dstmem size to one page

[Barry Song <21cnbao@gmail.com>]

On Wed, Dec 27, 2023 at 7:11 PM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> On 2023/12/27 09:07, Barry Song wrote:
> > On Wed, Dec 27, 2023 at 4:55 AM Chengming Zhou
> > <zhouchengming@bytedance.com> wrote:
> >>
> >> Change the dstmem size from 2 * PAGE_SIZE to only one page since
> >> we only need at most one page when compress, and the "dlen" is also
> >> PAGE_SIZE in acomp_request_set_params(). If the output size > PAGE_SIZE
> >> we don't wanna store the output in zswap anyway.
> >>
> >> So change it to one page, and delete the stale comment.
> >>
> >> There is no any history about the reason why we needed 2 pages, it has
> >> been 2 * PAGE_SIZE since the time zswap was first merged.
> >
> > i remember there was an over-compression case,  that means the compressed
> > data can be bigger than the source data. the similar thing is also done in zram
> > drivers/block/zram/zcomp.c
>
> Right, there is a buffer overflow report[1] that I just +to you.
>
> I think over-compression is all right, but buffer overflow is not acceptable,
> so we should fix any buffer overflow problem IMHO. Anyway, 2 pages maybe
> overflowed too, just with smaller probability, right?

practically, the typical page size is 4KB or above, so we have never seen 2
pages can be overflowed. We may have a chance to let CPU-based
compression code to return earlier before overflowing though it is still
very tough.
but for accelerators-based compression in drivers/crypto, the only choice is
giving its dma engine a buffer whose length is enough - 2*PAGE_SIZE.

so i don't think this patch is correct.

>
> Thanks.
>
> >
> > int zcomp_compress(struct zcomp_strm *zstrm,
> >                 const void *src, unsigned int *dst_len)
> > {
> >         /*
> >          * Our dst memory (zstrm->buffer) is always `2 * PAGE_SIZE' sized
> >          * because sometimes we can endup having a bigger compressed data
> >          * due to various reasons: for example compression algorithms tend
> >          * to add some padding to the compressed buffer. Speaking of padding,
> >          * comp algorithm `842' pads the compressed length to multiple of 8
> >          * and returns -ENOSP when the dst memory is not big enough, which
> >          * is not something that ZRAM wants to see. We can handle the
> >          * `compressed_size > PAGE_SIZE' case easily in ZRAM, but when we
> >          * receive -ERRNO from the compressing backend we can't help it
> >          * anymore. To make `842' happy we need to tell the exact size of
> >          * the dst buffer, zram_drv will take care of the fact that
> >          * compressed buffer is too big.
> >          */
> >         *dst_len = PAGE_SIZE * 2;
> >
> >         return crypto_comp_compress(zstrm->tfm,
> >                         src, PAGE_SIZE,
> >                         zstrm->buffer, dst_len);
> > }
> >
> >
> >>
> >> According to Yosry and Nhat, one potential reason is that we used to
> >> store a zswap header containing the swap entry in the compressed page
> >> for writeback purposes, but we don't do that anymore.
> >>
> >> This patch works good in kernel build testing even when the input data
> >> doesn't compress at all (i.e. dlen == PAGE_SIZE), which we can see
> >> from the bpftrace tool:
> >>
> >> bpftrace -e 'k:zpool_malloc {@[(uint32)arg1==4096]=count()}'
> >> @[1]: 2
> >> @[0]: 12011430
> >>
> >> Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
> >> Reviewed-by: Nhat Pham <nphamcs@gmail.com>
> >> Acked-by: Chris Li <chrisl@kernel.org> (Google)
> >> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> >> ---
> >>  mm/zswap.c | 5 ++---
> >>  1 file changed, 2 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/mm/zswap.c b/mm/zswap.c
> >> index 7ee54a3d8281..976f278aa507 100644
> >> --- a/mm/zswap.c
> >> +++ b/mm/zswap.c
> >> @@ -707,7 +707,7 @@ static int zswap_dstmem_prepare(unsigned int cpu)
> >>         struct mutex *mutex;
> >>         u8 *dst;
> >>
> >> -       dst = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL, cpu_to_node(cpu));
> >> +       dst = kmalloc_node(PAGE_SIZE, GFP_KERNEL, cpu_to_node(cpu));
> >>         if (!dst)
> >>                 return -ENOMEM;
> >>
> >> @@ -1662,8 +1662,7 @@ bool zswap_store(struct folio *folio)
> >>         sg_init_table(&input, 1);
> >>         sg_set_page(&input, page, PAGE_SIZE, 0);
> >>
> >> -       /* zswap_dstmem is of size (PAGE_SIZE * 2). Reflect same in sg_list */
> >> -       sg_init_one(&output, dst, PAGE_SIZE * 2);
> >> +       sg_init_one(&output, dst, PAGE_SIZE);
> >>         acomp_request_set_params(acomp_ctx->req, &input, &output, PAGE_SIZE, dlen);
> >>         /*
> >>          * it maybe looks a little bit silly that we send an asynchronous request,
> >>
> >> --
> >> b4 0.10.1
> >>

Thanks
Barry