Re: [PATCH v13 22/22] mm: zswap: Batched zswap_compress() with compress batching of large folios.

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Fri, Nov 14, 2025 at 06:43:21AM +0000, Sridhar, Kanchana P wrote:
> 
> > -----Original Message-----
> > From: Yosry Ahmed <yosry.ahmed@linux.dev>
> > Sent: Thursday, November 13, 2025 9:52 PM
> > To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
> > Cc: linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> > hannes@cmpxchg.org; nphamcs@gmail.com; chengming.zhou@linux.dev;
> > usamaarif642@gmail.com; ryan.roberts@arm.com; 21cnbao@gmail.com;
> > ying.huang@linux.alibaba.com; akpm@linux-foundation.org;
> > senozhatsky@chromium.org; sj@kernel.org; kasong@tencent.com; linux-
> > crypto@vger.kernel.org; herbert@gondor.apana.org.au;
> > davem@davemloft.net; clabbe@baylibre.com; ardb@kernel.org;
> > ebiggers@google.com; surenb@google.com; Accardi, Kristen C
> > <kristen.c.accardi@intel.com>; Gomes, Vinicius <vinicius.gomes@intel.com>;
> > Feghali, Wajdi K <wajdi.k.feghali@intel.com>; Gopal, Vinodh
> > <vinodh.gopal@intel.com>
> > Subject: Re: [PATCH v13 22/22] mm: zswap: Batched zswap_compress() with
> > compress batching of large folios.
> > 
> > On Thu, Nov 13, 2025 at 11:55:10PM +0000, Sridhar, Kanchana P wrote:
> > >
> > > > -----Original Message-----
> > > > From: Yosry Ahmed <yosry.ahmed@linux.dev>
> > > > Sent: Thursday, November 13, 2025 1:35 PM
> > > > To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
> > > > Cc: linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> > > > hannes@cmpxchg.org; nphamcs@gmail.com;
> > chengming.zhou@linux.dev;
> > > > usamaarif642@gmail.com; ryan.roberts@arm.com; 21cnbao@gmail.com;
> > > > ying.huang@linux.alibaba.com; akpm@linux-foundation.org;
> > > > senozhatsky@chromium.org; sj@kernel.org; kasong@tencent.com; linux-
> > > > crypto@vger.kernel.org; herbert@gondor.apana.org.au;
> > > > davem@davemloft.net; clabbe@baylibre.com; ardb@kernel.org;
> > > > ebiggers@google.com; surenb@google.com; Accardi, Kristen C
> > > > <kristen.c.accardi@intel.com>; Gomes, Vinicius
> > <vinicius.gomes@intel.com>;
> > > > Feghali, Wajdi K <wajdi.k.feghali@intel.com>; Gopal, Vinodh
> > > > <vinodh.gopal@intel.com>
> > > > Subject: Re: [PATCH v13 22/22] mm: zswap: Batched zswap_compress()
> > with
> > > > compress batching of large folios.
> > > >
> > [..]
> > > > > +		/*
> > > > > +		 * If a page cannot be compressed into a size smaller than
> > > > > +		 * PAGE_SIZE, save the content as is without a compression,
> > > > to
> > > > > +		 * keep the LRU order of writebacks.  If writeback is disabled,
> > > > > +		 * reject the page since it only adds metadata overhead.
> > > > > +		 * swap_writeout() will put the page back to the active LRU
> > > > list
> > > > > +		 * in the case.
> > > > > +		 *
> > > > > +		 * It is assumed that any compressor that sets the output
> > > > length
> > > > > +		 * to 0 or a value >= PAGE_SIZE will also return a negative
> > > > > +		 * error status in @err; i.e, will not return a successful
> > > > > +		 * compression status in @err in this case.
> > > > > +		 */
> > > >
> > > > Ugh, checking the compression error and checking the compression length
> > > > are now in separate places so we need to check if writeback is disabled
> > > > in separate places and store the page as-is. It's ugly, and I think the
> > > > current code is not correct.
> > >
> > > The code is 100% correct. You need to spend more time understanding
> > > the code. I have stated my assumption above in the comments to
> > > help in understanding the "why".
> > >
> > > From a maintainer, I would expect more responsible statements than
> > > this. A flippant remark made without understanding the code (and,
> > > disparaging the comments intended to help you do this), can impact
> > > someone's career. I am held accountable in my job based on your
> > > comments.
> > >
> > > That said, I have worked tirelessly and innovated to make the code
> > > compliant with Herbert's suggestions (which btw have enabled an
> > > elegant batching implementation and code commonality for IAA and
> > > software compressors), validated it thoroughly for IAA and ZSTD to
> > > ensure that both demonstrate performance improvements, which
> > > are crucial for memory savings. I am proud of this work.
> > >
> > >
> > > >
> > > > > +		if (err && !wb_enabled)
> > > > > +			goto compress_error;
> > > > > +
> > > > > +		for_each_sg(acomp_ctx->sg_outputs->sgl, sg, nr_comps, k) {
> > > > > +			j = k + i;
> > > >
> > > > Please use meaningful iterator names rather than i, j, and k and the huge
> > > > comment explaining what they are.
> > >
> > > I happen to have a different view: having longer iterator names firstly makes
> > > code seem "verbose" and detracts from readability, not to mention
> > exceeding the
> > > 80-character line limit. The comments are essential for code maintainability
> > > and avoid out-of-bounds errors when the next zswap developer wants to
> > > optimize the code.
> > >
> > > One drawback of i/j/k iterators is mis-typing errors which cannot be caught
> > > at compile time. Let me think some more about how to strike a good
> > balance.
> > >
> > > >
> > > > > +			dst = acomp_ctx->buffers[k];
> > > > > +			dlen = sg->length | *errp;
> > > >
> > > > Why are we doing this?
> > > >
> > > > > +
> > > > > +			if (dlen < 0) {
> > > >
> > > > We should do the incompressible page handling also if dlen is PAGE_SIZE,
> > > > or if the compression failed (I guess that's the intention of bit OR'ing
> > > > with *errp?)
> > >
> > > Yes, indeed: that's the intention of bit OR'ing with *errp.
> > 
> > ..and you never really answered my question. In the exising code we
> > store the page as incompressible if writeback is enabled AND
> > crypto_wait_req() fails or dlen is zero or PAGE_SIZE. We check above
> > if crypto_wait_req() fails and writeback is disabled, but what about the
> > rest?
> 
> Let me explain this some more. The new code only relies on the assumption
> that if dlen is zero or >= PAGE_SIZE, the compressor will not return a 0
> ("successful status"). In other words, the compressor will return an error status
> in this case, which is expected to be a negative error code.

I am not sure if all compressors do that, especially for the case where
dlen >= PAGE_SIZE. The existing code does not assume that there will be
an error code in these cases.

For the dlen == 0 case, the check was introduced recently by commit
dca4437a5861 ("mm/zswap: store <PAGE_SIZE compression failed page
as-is"). Looking through the history it seems like it was introduced in
v4 of that patch but I don't see the reasoning.

SeongJae, did you observe any compressors returning dlen == 0 but no
error code? What was the reasoning behind the dlen == 0 check?

> 
> Under these (hopefully valid) assumptions, the code handles the simple case
> of an error compression return status and writeback is disabled, by the
> "goto compress_error".
> 
> The rest is handled by these:
> 
> 1) First, I need to adapt the use of sg_outputs->sgl->length to represent the
> compress length for software compressors, so I do this after crypto_wait_req()
> returns:
> 
>                 acomp_ctx->sg_outputs->sgl->length = acomp_ctx->req->dlen;

For SW compressors, why is acomp_ctx->sg_outputs->sgl->length not set?
IIUC we are using the same API for SW and HW compressors, why is the
output length in different places for each of them?

> 
> I did not want to propose any changes to crypto software compressors protocols.
> 
> 2) After the check for the "if (err && !wb_enabled)" case, the new code has this:
> 
>                 for_each_sg(acomp_ctx->sg_outputs->sgl, sg, nr_comps, k) {
>                         j = k + i;
>                         dst = acomp_ctx->buffers[k];
>                         dlen = sg->length | *errp;
> 
>                         if (dlen < 0) {
>                                 dlen = PAGE_SIZE;
>                                 dst = kmap_local_page(folio_page(folio, start + j));
>                         }
> 
> For batching compressors, namely, iaa_crypto, the individual output SG
> lists sg->length follows the requirements from Herbert: each sg->length
> is the compressed length or the error status (a negative error code).
> 
> Then all I need to know whether to store the page as incompressible
> is to either directly test if sg->length is negative (for batching compressors),
> or sg->length bit-OR'ed with the crypto_wait_req() return status ("err")
> is negative. This is accomplished by the "dlen = sg->length | *errp;".
> 
> I believe this maintains backward compatibility with the existing code.
> Please let me know if you agree.

For batching compressors, will 'err' be set as well, or just sg->length?
If it's just sg->length, then we need to check again if WB is enabled
here before storing the page uncompressed. Right?

> 
> > 
> > We don't check again if writeback is enabled before storing the page is
> > incompressible, and we do not check if dlen is zero or PAGE_SIZE. Are
> > these cases no longer possible?
> 
> Hope the above explanation clarifies things some more? These case
> are possible, and as long as they return an error status, they should be
> correctly handled by the new code.

As mentioned above, I am not sure if that's correct for dlen >=
PAGE_SIZE.

> 
> > 
> > Also, why use errp, why not explicitly use the appropriate error code?
> > It's also unclear to me why the error code is always zero with HW
> > compression?
> 
> This is because of the sg->length requirements (compressed length/error)
> for the batching interface suggested by Herbert. Hence, I upfront define
> err_sg to 0, and, set errp to &err_sg for batching compressors. For software
> compressors, errp is set to &err, namely, the above check will always apply
> the software compressor's error status to the compressed length via
> the bit-OR to determine if the page needs to be stored uncompressed.

Thanks for the clarification. I understand that the error code has
different sources for SW and HW compressors, but I do not like using
errp as an indirection. It makes the code unclear. I would rather we
explicitly check err for SW compressors and dlen for HW compressors.

That being said, I thought what Herbert suggested was that the same API
is used for both SW and HW compressors. IOW, either way we submit a
batch of pages (8 pages for SW compressors), and then the crypto API
would either give the entire batch to the compressor if it supports
batching, or loop over them internally and hand them page-by-page to
the compressor.

This would simplify usage as we do not have to handle the differences in
zswap.

If that is not doable, at the very least the API should be consistent.
Right now the error code and length are propagated differently to the
caller based on whether or not the compressor support batching.

> 
> 
> > 
> > >
> > > >
> > > > > +				dlen = PAGE_SIZE;
> > > > > +				dst = kmap_local_page(folio_page(folio, start
> > > > + j));
> > > > > +			}
> > > > > +
> > > > > +			handle = zs_malloc(pool->zs_pool, dlen, gfp, nid);
> > > > >
> > > > > -	zs_obj_write(pool->zs_pool, handle, dst, dlen);
> > > > > -	entry->handle = handle;
> > > > > -	entry->length = dlen;
> > > > > +			if (IS_ERR_VALUE(handle)) {
> > > > > +				if (PTR_ERR((void *)handle) == -ENOSPC)
> > > > > +					zswap_reject_compress_poor++;
> > > > > +				else
> > > > > +					zswap_reject_alloc_fail++;
> > > > >
> > > > > -unlock:
> > > > > -	if (mapped)
> > > > > -		kunmap_local(dst);
> > > > > -	if (comp_ret == -ENOSPC || alloc_ret == -ENOSPC)
> > > > > -		zswap_reject_compress_poor++;
> > > > > -	else if (comp_ret)
> > > > > -		zswap_reject_compress_fail++;
> > > > > -	else if (alloc_ret)
> > > > > -		zswap_reject_alloc_fail++;
> > > > > +				goto err_unlock;
> > > > > +			}
> > > > > +
> > > > > +			zs_obj_write(pool->zs_pool, handle, dst, dlen);
> > > > > +			entries[j]->handle = handle;
> > > > > +			entries[j]->length = dlen;
> > > > > +			if (dst != acomp_ctx->buffers[k])
> > > > > +				kunmap_local(dst);
> > > > > +		}
> > > > > +	} /* finished compress and store nr_pages. */
> > > > > +
> > > > > +	mutex_unlock(&acomp_ctx->mutex);
> > > > > +	return true;
> > > > > +
> > > > > +compress_error:
> > > > > +	for_each_sg(acomp_ctx->sg_outputs->sgl, sg, nr_comps, k) {
> > > > > +		if ((int)sg->length < 0) {
> > > > > +			if ((int)sg->length == -ENOSPC)
> > > > > +				zswap_reject_compress_poor++;
> > > > > +			else
> > > > > +				zswap_reject_compress_fail++;
> > > > > +		}
> > > > > +	}
> > > > >
> > > > > +err_unlock:
> > > > >  	mutex_unlock(&acomp_ctx->mutex);
> > > > > -	return comp_ret == 0 && alloc_ret == 0;
> > > > > +	return false;
> > > > >  }
> > > > >
> > > > >  static bool zswap_decompress(struct zswap_entry *entry, struct folio
> > > > *folio)
> > > > > @@ -1488,12 +1604,9 @@ static bool zswap_store_pages(struct folio
> > > > *folio,
> > > > >  		INIT_LIST_HEAD(&entries[i]->lru);
> > > > >  	}
> > > > >
> > > > > -	for (i = 0; i < nr_pages; ++i) {
> > > > > -		struct page *page = folio_page(folio, start + i);
> > > > > -
> > > > > -		if (!zswap_compress(page, entries[i], pool, wb_enabled))
> > > > > -			goto store_pages_failed;
> > > > > -	}
> > > > > +	if (unlikely(!zswap_compress(folio, start, nr_pages, entries, pool,
> > > > > +				     nid, wb_enabled)))
> > > > > +		goto store_pages_failed;
> > > > >
> > > > >  	for (i = 0; i < nr_pages; ++i) {
> > > > >  		struct zswap_entry *old, *entry = entries[i];
> > > > > --
> > > > > 2.27.0
> > > > >