On Sat 29-09-18 04:46:09, Jerome Glisse wrote: > On Fri, Sep 28, 2018 at 07:28:16PM -0700, John Hubbard wrote: > > Actually, the latest direction on that discussion was toward periodically > > writing back, even while under RDMA, via bounce buffers: > > > > https://lkml.kernel.org/r/20180710082100.mkdwngdv5kkrcz6n@xxxxxxxxxxxxxx > > > > I still think that's viable. Of course, there are other things besides > > writeback (see below) that might also lead to waiting. > > Write back under bounce buffer is fine, when looking back at links you > provided the solution that was discuss was blocking in page_mkclean() > which is horrible in my point of view. Yeah, after looking into it for some time, we figured that waiting for page pins in page_mkclean() isn't really going to fly due to deadlocks. So we came up with the bounce buffers idea which should solve that nicely. > > > With the solution put forward here you can potentialy wait _forever_ for > > > the driver that holds a pin to drop it. This was the point i was trying to > > > get accross during LSF/MM. > > > > I agree that just blocking indefinitely is generally unacceptable for kernel > > code, but we can probably avoid it for many cases (bounce buffers), and > > if we think it is really appropriate (file system unmounting, maybe?) then > > maybe tolerate it in some rare cases. > > > > >You can not fix broken hardware that decided to > > > use GUP to do a feature they can't reliably do because their hardware is > > > not capable to behave. > > > > > > Because code is easier here is what i was meaning: > > > > > > https://cgit.freedesktop.org/~glisse/linux/commit/?h=gup&id=a5dbc0fe7e71d347067579f13579df372ec48389 > > > https://cgit.freedesktop.org/~glisse/linux/commit/?h=gup&id=01677bc039c791a16d5f82b3ef84917d62fac826 > > > > > > > While that may work sometimes, I don't think it is reliable enough to trust for > > identifying pages that have been gup-pinned. There's just too much overloading of > > other mechanisms going on there, and if we pile on top with this constraint of "if you > > have +3 refcounts, and this particular combination of page counts and mapcounts, then > > you're definitely a long-term pinned page", I think users will find a lot of corner > > cases for us that break that assumption. > > So the mapcount == refcount (modulo extra reference for mapping and > private) should holds, here are the case when it does not: > - page being migrated > - page being isolated from LRU > - mempolicy changes against the page > - page cache lookup > - some file system activities > - i likely miss couples here i am doing that from memory > > What matter is that all of the above are transitory, the extra reference > only last for as long as it takes for the action to finish (migration, > mempolicy change, ...). > > So skipping those false positive page while reclaiming likely make sense, > the blocking free buffer maybe not. Well, as John wrote, these page refcount are fragile (and actually filesystem dependent as some filesystems hold page reference from their page->private data and some don't). So I think we really need a new reliable mechanism for tracking page references from GUP. And John works towards that. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
