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Message-ID: <CAGsJ_4zMGEEUXswFs0VePye5wzvSt94JkrTvypHAV4G2v1XOkA@mail.gmail.com> Date: Wed, 27 Dec 2023 19:32:22 +1300 From: Barry Song <21cnbao@...il.com> To: Chengming Zhou <zhouchengming@...edance.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, Seth Jennings <sjenning@...hat.com>, Johannes Weiner <hannes@...xchg.org>, Vitaly Wool <vitaly.wool@...sulko.com>, Nhat Pham <nphamcs@...il.com>, Chris Li <chriscli@...gle.com>, Yosry Ahmed <yosryahmed@...gle.com>, Dan Streetman <ddstreet@...e.org>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, Chris Li <chrisl@...nel.org> Subject: Re: [PATCH v4 1/6] mm/zswap: change dstmem size to one page On Wed, Dec 27, 2023 at 7:11 PM Chengming Zhou <zhouchengming@...edance.com> wrote: > > On 2023/12/27 09:07, Barry Song wrote: > > On Wed, Dec 27, 2023 at 4:55 AM Chengming Zhou > > <zhouchengming@...edance.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@...gle.com> > >> Reviewed-by: Nhat Pham <nphamcs@...il.com> > >> Acked-by: Chris Li <chrisl@...nel.org> (Google) > >> Signed-off-by: Chengming Zhou <zhouchengming@...edance.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
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