Re: [RESEND PATCH v2] fs/isofs: Replace kmap() with kmap_local_page()

[Jan Kara]

On Sun 21-08-22 19:50:12, Fabio M. De Francesco wrote:
> The use of kmap() is being deprecated in favor of kmap_local_page().
> 
> There are two main problems with kmap(): (1) It comes with an overhead as
> mapping space is restricted and protected by a global lock for
> synchronization and (2) it also requires global TLB invalidation when the
> kmap’s pool wraps and it might block when the mapping space is fully
> utilized until a slot becomes available.
> 
> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).
> Tasks can be preempted and, when scheduled to run again, the kernel
> virtual addresses are restored and still valid. It is faster than kmap()
> in kernels with HIGHMEM enabled.
> 
> Since kmap_local_page() can be safely used in compress.c, it should be
> called everywhere instead of kmap().
> 
> Therefore, replace kmap() with kmap_local_page() in compress.c. Where it
> is needed, use memzero_page() instead of open coding kmap_local_page()
> plus memset() to fill the pages with zeros. Delete the redundant
> flush_dcache_page() in the two call sites of memzero_page().
> 
> Tested with mkisofs on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel
> with HIGHMEM64GB enabled.
> 
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>

Thanks for the patch! It looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
> 
> Some days ago Andrew requested a resend of this patch.[1]
> 
> v1->v2: Cast zisofs_sink_page to pointer to unsigned char.
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> 
> Many thanks to Jan Kara for the comments and suggestions provided with
> replying to my previous RFC.[2] Furthermore, I want to thank Ira Weiny for
> the advices he provided, especially about how to use mkisofs to test that
> this patch is working properly.
> 
> [1] https://lore.kernel.org/all/20220801122709.8164-1-fmdefrancesco@xxxxxxxxx/
> [2] https://lore.kernel.org/lkml/20220726145024.rryvw7ot7j2c6tqv@quack3/
> 
>  fs/isofs/compress.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/isofs/compress.c b/fs/isofs/compress.c
> index 95a19f25d61c..107007c38d08 100644
> --- a/fs/isofs/compress.c
> +++ b/fs/isofs/compress.c
> @@ -67,8 +67,7 @@ static loff_t zisofs_uncompress_block(struct inode *inode, loff_t block_start,
>  		for ( i = 0 ; i < pcount ; i++ ) {
>  			if (!pages[i])
>  				continue;
> -			memset(page_address(pages[i]), 0, PAGE_SIZE);
> -			flush_dcache_page(pages[i]);
> +			memzero_page(pages[i], 0, PAGE_SIZE);
>  			SetPageUptodate(pages[i]);
>  		}
>  		return ((loff_t)pcount) << PAGE_SHIFT;
> @@ -120,7 +119,7 @@ static loff_t zisofs_uncompress_block(struct inode *inode, loff_t block_start,
>  	       zerr != Z_STREAM_END) {
>  		if (!stream.avail_out) {
>  			if (pages[curpage]) {
> -				stream.next_out = page_address(pages[curpage])
> +				stream.next_out = kmap_local_page(pages[curpage])
>  						+ poffset;
>  				stream.avail_out = PAGE_SIZE - poffset;
>  				poffset = 0;
> @@ -176,6 +175,10 @@ static loff_t zisofs_uncompress_block(struct inode *inode, loff_t block_start,
>  				flush_dcache_page(pages[curpage]);
>  				SetPageUptodate(pages[curpage]);
>  			}
> +			if (stream.next_out != (unsigned char *)zisofs_sink_page) {
> +				kunmap_local(stream.next_out);
> +				stream.next_out = NULL;
> +			}
>  			curpage++;
>  		}
>  		if (!stream.avail_in)
> @@ -183,6 +186,8 @@ static loff_t zisofs_uncompress_block(struct inode *inode, loff_t block_start,
>  	}
>  inflate_out:
>  	zlib_inflateEnd(&stream);
> +	if (stream.next_out && stream.next_out != (unsigned char *)zisofs_sink_page)
> +		kunmap_local(stream.next_out);
>  
>  z_eio:
>  	mutex_unlock(&zisofs_zlib_lock);
> @@ -283,9 +288,7 @@ static int zisofs_fill_pages(struct inode *inode, int full_page, int pcount,
>  	}
>  
>  	if (poffset && *pages) {
> -		memset(page_address(*pages) + poffset, 0,
> -		       PAGE_SIZE - poffset);
> -		flush_dcache_page(*pages);
> +		memzero_page(*pages, poffset, PAGE_SIZE - poffset);
>  		SetPageUptodate(*pages);
>  	}
>  	return 0;
> @@ -343,10 +346,8 @@ static int zisofs_read_folio(struct file *file, struct folio *folio)
>  	for (i = 0; i < pcount; i++, index++) {
>  		if (i != full_page)
>  			pages[i] = grab_cache_page_nowait(mapping, index);
> -		if (pages[i]) {
> +		if (pages[i])
>  			ClearPageError(pages[i]);
> -			kmap(pages[i]);
> -		}
>  	}
>  
>  	err = zisofs_fill_pages(inode, full_page, pcount, pages);
> @@ -357,7 +358,6 @@ static int zisofs_read_folio(struct file *file, struct folio *folio)
>  			flush_dcache_page(pages[i]);
>  			if (i == full_page && err)
>  				SetPageError(pages[i]);
> -			kunmap(pages[i]);
>  			unlock_page(pages[i]);
>  			if (i != full_page)
>  				put_page(pages[i]);
> -- 
> 2.37.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR