Re: [PATCH] btrfs: fix the memory leak caused in lzo_compress_pages()

[David Sterba <dsterba@suse.cz>]

On Sat, Nov 20, 2021 at 04:34:11PM +0800, Qu Wenruo wrote:
> [BUG]
> Fstests generic/027 is pretty easy to trigger a slow but steady memory
> leak if run with "-o compress=lzo" mount option.
> 
> Normally one single run of generic/027 is enough to eat up at least 4G ram.
> 
> [CAUSE]
> In commit d4088803f511 ("btrfs: subpage: make lzo_compress_pages()
> compatible") we changed how @page_in is released.
> 
> But that refactor makes @page_in only released after all pages being
> compressed.
> 
> This leaves error path not releasing @page_in. And by "error path"
> things like incompressible data will also be treated as an error
> (-E2BIG).
> 
> Thus it can leave btrfs to leak memory even there is nothing wrong
> happened.
> 
> [FIX]
> Add check under @out label to release @page_in when needed, so when we
> hit any error, the input page is properly released.
> 
> Reported-by: Josef Bacik <josef@toxicpanda.com>
> Fixes: d4088803f511 ("btrfs: subpage: make lzo_compress_pages() compatible")

This commit has caused problems I don't want to repeat, it's an example
of a 'rewrite in one go' vs 'incremental changes', this is a pattern
known to avoid exactly because of the sublte semantics not transferred
into the new version. Please keep that in mind for future patches, it's
much safer to review 5 patches doing the change in smaller steps.
Anyway, thanks for the fix, added to misc-next.