On Tue 11-12-18 11:08:53, Josef Bacik wrote: > On Tue, Dec 11, 2018 at 10:40:34AM +0100, Jan Kara wrote: > > > The lock_page_or_retry() case in particular gets hit a lot with > > > multi-threaded applications that got paged out because of heavy memory > > > pressure. By no means is it as high as just the normal readpage or > > > readahead cases, but it's not 0, so I'd rather have the extra helper here > > > to make sure we're never getting screwed. > > > > Do you mean the case where we the page is locked in filemap_fault() (so > > that lock_page_or_retry() bails after waiting) and when the page becomes > > unlocked it is not uptodate? Because that is the reason why you opencode > > lock_page_or_retry(), right? I'm not aware of any normal code path that > > would create page in page cache and not try to fill it with data before > > unlocking it so that's why I'm really trying to make sure we understand > > each other. > > Uhh so that's embarressing. We have an internal patchset that I thought > was upstream but hasn't come along yet. Basically before this patchset > the way we dealt with this problem was to short-circuit readahead IO's by > checking to see if the blkcg was congested (or if there was a fatal > signal pending) and doing bio_wouldblock_error on the bio. So this very > case came up a lot, readahead would go through because it got in before > we were congested, but would then get throttled, and then once the > throttling was over would get aborted. Other threads would run into > these pages that had been locked, but they are never read in which means > they waited for the lock to be dropped, did the VM_FAULT_RETRY, came back > unable to drop the mmap_sem and did the actual readpage() and would get > throttled. OK, I'm somewhat unsure why we throttle on bios that actually get aborted but that's a separate discussion over a different patches. Overall it makes sense that some submitted readahead may actually get aborted on congestion and thus unlocked pages will not be uptodate. So I agree that this case is actually reasonably likely to happen. Just please mention case like aborted readahead in the comment so that we don't wonder about the reason in a few years again. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
