Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O

[Michal Hocko]

[Cc fs-devel]

On Wed 30-01-19 13:44:19, Vlastimil Babka wrote:
> From: Jiri Kosina <jkosina@xxxxxxx>
> 
> preadv2(RWF_NOWAIT) can be used to open a side-channel to pagecache contents, as
> it reveals metadata about residency of pages in pagecache.
> 
> If preadv2(RWF_NOWAIT) returns immediately, it provides a clear "page not
> resident" information, and vice versa.
> 
> Close that sidechannel by always initiating readahead on the cache if we
> encounter a cache miss for preadv2(RWF_NOWAIT); with that in place, probing
> the pagecache residency itself will actually populate the cache, making the
> sidechannel useless.

I guess the current wording doesn't disallow background IO to be
triggered for EAGAIN case. I am not sure whether that breaks clever
applications which try to perform larger IO for those cases though.

> Originally-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> Cc: Kevin Easton <kevin@xxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Cyril Hrubis <chrubis@xxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> Cc: Daniel Gruss <daniel@xxxxxxxx>
> Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/filemap.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 9f5e323e883e..7bcdd36e629d 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2075,8 +2075,6 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  
>  		page = find_get_page(mapping, index);
>  		if (!page) {
> -			if (iocb->ki_flags & IOCB_NOWAIT)
> -				goto would_block;
>  			page_cache_sync_readahead(mapping,
>  					ra, filp,
>  					index, last_index - index);

Maybe a stupid question but I am not really familiar with this path but
what exactly does prevent a sync read down page_cache_sync_readahead
path?
-- 
Michal Hocko
SUSE Labs