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SubjectRe: [RFC PATCH] btrfs: Replace kmap() with kmap_local_page() in zlib.c
On Tue, Jun 14, 2022 at 12:47:18PM +0200, Fabio M. De Francesco wrote:

[snip]

> @@ -148,26 +148,26 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
> int i;
>
> for (i = 0; i < in_buf_pages; i++) {
> - if (in_page) {
> - kunmap(in_page);
> + if (data_in) {
> + kunmap_local(data_in);
> put_page(in_page);
> }
> in_page = find_get_page(mapping,
> start >> PAGE_SHIFT);
> - data_in = kmap(in_page);
> + data_in = kmap_local_page(in_page);
> memcpy(workspace->buf + i * PAGE_SIZE,
> data_in, PAGE_SIZE);
> start += PAGE_SIZE;

This is moving start forward...

> }
> workspace->strm.next_in = workspace->buf;
> } else {
> - if (in_page) {
> - kunmap(in_page);
> + if (data_in) {
> + kunmap_local(data_in);
> put_page(in_page);
> }
> in_page = find_get_page(mapping,
> start >> PAGE_SHIFT);
> - data_in = kmap(in_page);
> + data_in = kmap_local_page(in_page);
> start += PAGE_SIZE;
> workspace->strm.next_in = data_in;
> }
> @@ -196,9 +196,14 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
> * the stream end if required
> */
> if (workspace->strm.avail_out == 0) {
> - kunmap(out_page);
> + kunmap_local(data_in);
> + data_in = NULL;
> + put_page(in_page);

I don't think you need to put/get the page to do the remapping.

> +
> + kunmap_local(cpage_out);
> if (nr_pages == nr_dest_pages) {
> - out_page = NULL;
> + cpage_out = NULL;
> + put_page(out_page);
> ret = -E2BIG;
> goto out;
> }
> @@ -207,7 +212,14 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
> ret = -ENOMEM;
> goto out;
> }
> - cpage_out = kmap(out_page);
> + cpage_out = kmap_local_page(out_page);
> +
> + in_page = find_get_page(mapping, start >> PAGE_SHIFT);

I think start is the wrong value here. It got incremented above.

> + data_in = kmap_local_page(in_page);
> + workspace->strm.next_in = data_in;
> + workspace->strm.avail_in = min(bytes_left,
> + (unsigned long)workspace->buf_size);

I'm not sure about changing avail_in here either because nothing has happened
with the in buffer whilst doing this re-mapping AFAICS.

[snip]

I think the issue with the tests is contained to zlib_compress_pages().
But an idea to verify that is to split this patch between the changes in
zlib_compress_pages() [above] and zlib_decompress_bio() [below] because below
looks very straight forward.

Ira

> @@ -287,7 +311,7 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
> unsigned long buf_start;
> struct page **pages_in = cb->compressed_pages;
>
> - data_in = kmap(pages_in[page_in_index]);
> + data_in = kmap_local_page(pages_in[page_in_index]);
> workspace->strm.next_in = data_in;
> workspace->strm.avail_in = min_t(size_t, srclen, PAGE_SIZE);
> workspace->strm.total_in = 0;
> @@ -309,7 +333,7 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>
> if (Z_OK != zlib_inflateInit2(&workspace->strm, wbits)) {
> pr_warn("BTRFS: inflateInit failed\n");
> - kunmap(pages_in[page_in_index]);
> + kunmap_local(data_in);
> return -EIO;
> }
> while (workspace->strm.total_in < srclen) {
> @@ -336,13 +360,14 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>
> if (workspace->strm.avail_in == 0) {
> unsigned long tmp;
> - kunmap(pages_in[page_in_index]);
> +
> + kunmap_local(data_in);
> page_in_index++;
> if (page_in_index >= total_pages_in) {
> data_in = NULL;
> break;
> }
> - data_in = kmap(pages_in[page_in_index]);
> + data_in = kmap_local_page(pages_in[page_in_index]);
> workspace->strm.next_in = data_in;
> tmp = srclen - workspace->strm.total_in;
> workspace->strm.avail_in = min(tmp, PAGE_SIZE);
> @@ -355,7 +380,7 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
> done:
> zlib_inflateEnd(&workspace->strm);
> if (data_in)
> - kunmap(pages_in[page_in_index]);
> + kunmap_local(data_in);
> if (!ret)
> zero_fill_bio(cb->orig_bio);
> return ret;
> --
> 2.36.1
>

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