Re: [PATCH v4] mm: don't cap request size based on read-ahead setting

[Jens Axboe <axboe@kernel.dk>]

On 11/18/2016 11:02 AM, Johannes Weiner wrote:
> On Thu, Nov 17, 2016 at 02:23:10PM -0700, Jens Axboe wrote:
>> We ran into a funky issue, where someone doing 256K buffered reads saw
>> 128K requests at the device level. Turns out it is read-ahead capping
>> the request size, since we use 128K as the default setting. This doesn't
>> make a lot of sense - if someone is issuing 256K reads, they should see
>> 256K reads, regardless of the read-ahead setting, if the underlying
>> device can support a 256K read in a single command.
>>
>> To make matters more confusing, there's an odd interaction with the
>> fadvise hint setting. If we tell the kernel we're doing sequential IO on
>> this file descriptor, we can get twice the read-ahead size. But if we
>> tell the kernel that we are doing random IO, hence disabling read-ahead,
>> we do get nice 256K requests at the lower level. This is because
>> ondemand and forced read-ahead behave differently, with the latter doing
>> the right thing. An application developer will be, rightfully,
>> scratching his head at this point, wondering wtf is going on. A good one
>> will dive into the kernel source, and silently weep.
>
> With the FADV_RANDOM part of the changelog updated, this looks good to
> me. Just a few nitpicks below.
>
>> This patch introduces a bdi hint, io_pages. This is the soft max IO size
>> for the lower level, I've hooked it up to the bdev settings here.
>> Read-ahead is modified to issue the maximum of the user request size,
>> and the read-ahead max size, but capped to the max request size on the
>> device side. The latter is done to avoid reading ahead too much, if the
>> application asks for a huge read. With this patch, the kernel behaves
>> like the application expects.
>>
>> Signed-off-by: Jens Axboe <axboe@fb.com>
>
>> @@ -207,12 +207,17 @@ int __do_page_cache_readahead(struct address_space
>> *mapping, struct file *filp,
>>   * memory at once.
>>   */
>>  int force_page_cache_readahead(struct address_space *mapping, struct file
>> *filp,
>
> Linewrap (but you already knew that ;))

Yeah, dunno wtf happened there...

>> -		pgoff_t offset, unsigned long nr_to_read)
>> +		               pgoff_t offset, unsigned long nr_to_read)
>>  {
>> +	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
>> +	struct file_ra_state *ra = &filp->f_ra;
>> +	unsigned long max_pages;
>> +
>>  	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
>>  		return -EINVAL;
>>
>> -	nr_to_read = min(nr_to_read, inode_to_bdi(mapping->host)->ra_pages);
>> +	max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
>> +	nr_to_read = min(nr_to_read, max_pages);
>
> It would be useful to have the comment on not capping below optimal IO
> size from ondemand_readahead() here as well.

Good idea, I'll copy/paste the comment from the ondemand part.

>> @@ -369,10 +374,18 @@ ondemand_readahead(struct address_space *mapping,
>>  		   bool hit_readahead_marker, pgoff_t offset,
>>  		   unsigned long req_size)
>>  {
>> -	unsigned long max = ra->ra_pages;
>> +	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
>> +	unsigned long max_pages = ra->ra_pages;
>>  	pgoff_t prev_offset;
>>
>>  	/*
>> +	 * If the request exceeds the readahead window, allow the read to
>> +	 * be up to the optimal hardware IO size
>> +	 */
>> +	if (req_size > max_pages && bdi->io_pages > max_pages)
>> +		max_pages = min(req_size, bdi->io_pages);
>> +
>> +	/*
>>  	 * start of file
>>  	 */
>>  	if (!offset)
>
> Please feel free to add:
>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Thanks!

-- 
Jens Axboe

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