Re: [Question] Should direct reclaim time be bounded?

[Vlastimil Babka]

On 4/23/19 6:39 PM, Mike Kravetz wrote:
>> That being said, I do not think __GFP_RETRY_MAYFAIL is wrong here. It
>> looks like there is something wrong in the reclaim going on.
> 
> Ok, I will start digging into that.  Just wanted to make sure before I got
> into it too deep.
> 
> BTW - This is very easy to reproduce.  Just try to allocate more huge pages
> than will fit into memory.  I see this 'reclaim taking forever' behavior on
> v5.1-rc5-mmotm-2019-04-19-14-53.  Looks like it was there in v5.0 as well.

I'd suspect this in should_continue_reclaim():

        /* Consider stopping depending on scan and reclaim activity */
        if (sc->gfp_mask & __GFP_RETRY_MAYFAIL) {
                /*
                 * For __GFP_RETRY_MAYFAIL allocations, stop reclaiming if the
                 * full LRU list has been scanned and we are still failing
                 * to reclaim pages. This full LRU scan is potentially
                 * expensive but a __GFP_RETRY_MAYFAIL caller really wants to succeed
                 */
                if (!nr_reclaimed && !nr_scanned)
                        return false;

And that for some reason, nr_scanned never becomes zero. But it's hard
to figure out through all the layers of functions :/