Re: [PATCH 2/8] reiserfs: use kmap_local_folio() in _get_block_create_0()

[Ira Weiny <ira.weiny@intel.com>]

On Sat, Dec 17, 2022 at 07:07:12PM +0000, Matthew Wilcox wrote:
> On Sat, Dec 17, 2022 at 09:14:05AM -0800, Ira Weiny wrote:
> > On Fri, Dec 16, 2022 at 08:53:41PM +0000, Matthew Wilcox (Oracle) wrote:
> > > Switch from the deprecated kmap() to kmap_local_folio().  For the
> > > kunmap_local(), I subtract off 'chars' to prevent the possibility that
> > > p has wrapped into the next page.
> > 
> > Thanks for tackling this one.  I think the conversion is mostly safe because I
> > don't see any reason the mapping is passed to another thread.
> > 
> > But comments like this make me leary:
> > 
> >          "But, this means the item might move if kmap schedules"
> > 
> > What does that mean?  That seems to imply there is something wrong with the
> > base code separate from the kmapping.
> 
> I should probably have deleted that comment.  I'm pretty sure what it
> refers to is that we don't hold a lock that prevents the item from
> moving.  When ReiserFS was written, we didn't have CONFIG_PREEMPT, so 
> if kmap() scheduled, that was a point at which the item could move.
> I don't think I introduced any additional brokenness by converting
> from kmap() to kmap_local().

Agreed.  I only said mostly safe because of the question about chars below.

> Maybe I'm wrong and somebody who
> understands ReiserFS can explain.

Yep.

> 
> > To the patch, I think subtracting chars might be an issue.  If chars > offset
> > and the loop takes the first 'if (done) break;' path then p will end up
> > pointing at the previous page wouldn't it?
> 
> I thought about that and managed to convince myself that chars was
> always < offset.  But now I'm not sure again.  Easiest way to fix
> this is actually to move the p += chars before the if (done) break;.

I thought about that too.  So yea that is fine.

> 
> I also need to rev this patch because it assumes that b_folio is a
> single page.
> 
> diff --git a/fs/reiserfs/inode.c b/fs/reiserfs/inode.c
> index 008855ddb365..be13ce7a38e1 100644
> --- a/fs/reiserfs/inode.c
> +++ b/fs/reiserfs/inode.c
> @@ -295,7 +295,6 @@ static int _get_block_create_0(struct inode *inode, sector_t block,
>  	int ret;
>  	int result;
>  	int done = 0;
> -	unsigned long offset;
>  
>  	/* prepare the key to look for the 'block'-th block of file */
>  	make_cpu_key(&key, inode,
> @@ -380,17 +379,16 @@ static int _get_block_create_0(struct inode *inode, sector_t block,
>  		set_buffer_uptodate(bh_result);
>  		goto finished;
>  	}
> -	/* read file tail into part of page */
> -	offset = (cpu_key_k_offset(&key) - 1) & (PAGE_SIZE - 1);
>  	copy_item_head(&tmp_ih, ih);
>  
>  	/*
>  	 * we only want to kmap if we are reading the tail into the page.
>  	 * this is not the common case, so we don't kmap until we are
> -	 * sure we need to.  But, this means the item might move if
> -	 * kmap schedules
> +	 * sure we need to.
>  	 */
> -	p = kmap_local_folio(bh_result->b_folio, offset);
> +	p = kmap_local_folio(bh_result->b_folio,
> +			offset_in_folio(bh_result->b_folio,
> +					cpu_key_k_offset(&key) - 1));

Yes good addition.

With these changes:

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

>  	memset(p, 0, inode->i_sb->s_blocksize);
>  	do {
>  		if (!is_direct_le_ih(ih)) {
> @@ -413,12 +411,11 @@ static int _get_block_create_0(struct inode *inode, sector_t block,
>  			chars = ih_item_len(ih) - path.pos_in_item;
>  		}
>  		memcpy(p, ih_item_body(bh, ih) + path.pos_in_item, chars);
> +		p += chars;
>  
>  		if (done)
>  			break;
>  
> -		p += chars;
> -
>  		/*
>  		 * we done, if read direct item is not the last item of
>  		 * node FIXME: we could try to check right delimiting key