On Tue, Sep 18, 2018 at 7:48 PM Um, Taeil <taeilum@xxxxxxxxxx> wrote: > > We can tell whether compressed size is greater than PAGE_SIZE by looking at the returned *dlen value from crypto_comp_compress. This should be fairly easy. > This is actually what zram is doing today. zram looks for *dlen and not take the compressed result if *dlen is greater than certain size. > I think errors from crypto_comp_compress should be real errors. > > Today in kernel compression drivers such as lzo and lz4, they do not stop just because compression result size is greater than source size. > Also, some H/W accelerators would not have the option of stopping compression just because of the result size is greater than source size. do you have a specific example of how this causes any actual problem? personally, i'd prefer reducing zswap_dstmem down to 1 page to save memory, since there is no case where zswap would ever want to use a compressed page larger than that. > > Thank you, > Taeil > > On 9/18/18, 2:44 PM, "Dan Streetman" <ddstreet@xxxxxxxx> wrote: > > On Tue, Sep 18, 2018 at 2:52 PM Um, Taeil <taeilum@xxxxxxxxxx> wrote: > > > > Problem statement: > > "compressed data are not fully copied to destination buffer when compressed data size is greater than source data" > > > > Why: > > 5th argument of crypto_comp_compress function is *dlen, which tell the compression driver how many bytes the destination buffer space is allocated (allowed to write data). > > This *dlen is important especially for H/W accelerator based compression driver because it is dangerous if we allow the H/W accelerator to access memory beyond *dst + *dlen. > > Note that buffer location would be passed as physical address. > > Due to the above reason, H/W accelerator based compression driver need to honor *dlen value when it serves crypto_comp_compress API. > > and that's exactly what zswap wants to happen - any compressor (hw or > sw) should fail with an error code (ENOSPC makes the most sense, but > zswap doesn't actually care) if the compressed data size is larger > than the provided data buffer. > > > Today, we pass slen = PAGE_SIZE and *dlen=PAGE_SIZE to crypto_comp_compress in zswap.c. > > If compressed data size is greater than source (uncompressed) data size, H/W accelerator cannot copy (deliver) the entire compressed data. > > If the "compressed" data is larger than 1 page, then there is no point > in storing the page in zswap. > > remember that zswap is different than zram; in zram, there's no other > place to store the data. However, with zswap, if compression fails or > isn't good, we can just pass the uncompressed page down to the swap > device. > > > > > Thank you, > > Taeil > > > > On 9/18/18, 7:15 AM, "Dan Streetman" <ddstreet@xxxxxxxx> wrote: > > > > On Mon, Sep 17, 2018 at 7:10 PM Um, Taeil <taeilum@xxxxxxxxxx> wrote: > > > > > > Currently, we allocate PAGE_SIZE * 2 for zswap_dstmem which is used as compression destination buffer. > > > > > > However, we pass only half of the size (PAGE_SIZE) to crypto_comp_compress. > > > > > > This might not be a problem for CPU based existing lzo, lz4 crypto compression driver implantation. > > > > > > However, this could be a problem for some H/W acceleration compression drivers, which honor destination buffer size when it prepares H/W resources. > > > > How exactly could it be a problem? > > > > > > > > Actually, this patch is aligned with what zram is passing when it calls crypto_comp_compress. > > > > > > The following simple patch will solve this problem. I tested it with existing crypto/lzo.c and crypto/lz4.c compression driver and it works fine. > > > > > > > > > > > > > > > > > > --- mm/zswap.c.orig 2018-09-14 14:36:37.984199232 -0700 > > > > > > +++ mm/zswap.c 2018-09-14 14:36:53.340189681 -0700 > > > > > > @@ -1001,7 +1001,7 @@ static int zswap_frontswap_store(unsigne > > > > > > struct zswap_entry *entry, *dupentry; > > > > > > struct crypto_comp *tfm; > > > > > > int ret; > > > > > > - unsigned int hlen, dlen = PAGE_SIZE; > > > > > > + unsigned int hlen, dlen = PAGE_SIZE * 2; > > > > > > unsigned long handle, value; > > > > > > char *buf; > > > > > > u8 *src, *dst; > > > > > > > > > > > > > > > > > > > > > > > > Thank you, > > > > > > Taeil > > > > > > > > > > > > > > >
