> do you have a specific example of how this causes any actual problem?
Yes, I have a H/W accelerator that tries to finish compression even if compressed data size is greater than source data size.
> 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.
I don't think "reducing zswap_dstmem down to 1 page" works in today's lzo and lz4 kernel implementation.
If I read kernel's lzo, lz4 code correctly, it does not stop compression when compressed data size is greater than source data size.
On 9/19/18, 8:47 AM, "Dan Streetman" <ddstreet@xxxxxxxx> wrote:
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
> > >
> > >
> >
> >
> >
>
>
>
