Re: [PATCH 5/5] btrfs: Increase buffer size for zlib functions

[Zaslonko Mikhail]

Hello,

On 26.11.2019 16:52, Josef Bacik wrote:
> On Tue, Nov 26, 2019 at 03:41:30PM +0100, Mikhail Zaslonko wrote:
>> Due to the small size of zlib buffer (1 page) set in btrfs code, s390
>> hardware compression is rather limited in terms of performance. Increasing
>> the buffer size to 4 pages would bring significant benefit for s390
>> hardware compression (up to 60% better performance compared to the
>> PAGE_SIZE buffer) and should not bring much overhead in terms of memory
>> consumption due to order 2 allocations.
>>
>> Signed-off-by: Mikhail Zaslonko <zaslonko@xxxxxxxxxxxxx>
> 
> We may have to make these allocations under memory pressure in the IO context,
> order 2 allocations here is going to be not awesome.  If you really want it then
> you need to at least be able to fall back to single page if you fail to get the
> allocation.  Thanks,
> 

As far as I understand GFP_KERNEL allocations would never fail for the order <= 
PAGE_ALLOC_COSTLY_ORDER. How else can the memory pressure condition be identified
here?

> josef
>