On Mon, Dec 17, 2018 at 10:34:43AM -0800, Matthew Wilcox wrote: > On Mon, Dec 17, 2018 at 01:11:50PM -0500, Jerome Glisse wrote: > > On Mon, Dec 17, 2018 at 08:58:19AM +1100, Dave Chinner wrote: > > > Sure, that's a possibility, but that doesn't close off any race > > > conditions because there can be DMA into the page in progress while > > > the page is being bounced, right? AFAICT this ext3+DIF/DIX case is > > > different in that there is no 3rd-party access to the page while it > > > is under IO (ext3 arbitrates all access to it's metadata), and so > > > nothing can actually race for modification of the page between > > > submission and bouncing at the block layer. > > > > > > In this case, the moment the page is unlocked, anyone else can map > > > it and start (R)DMA on it, and that can happen before the bio is > > > bounced by the block layer. So AFAICT, block layer bouncing doesn't > > > solve the problem of racing writeback and DMA direct to the page we > > > are doing IO on. Yes, it reduces the race window substantially, but > > > it doesn't get rid of it. > > > > So the event flow is: > > - userspace create object that match a range of virtual address > > against a given kernel sub-system (let's say infiniband) and > > let's assume that the range is an mmap() of a regular file > > - device driver do GUP on the range (let's assume it is a write > > GUP) so if the page is not already map with write permission > > in the page table than a page fault is trigger and page_mkwrite > > happens > > - Once GUP return the page to the device driver and once the > > device driver as updated the hardware states to allow access > > to this page then from that point on hardware can write to the > > page at _any_ time, it is fully disconnected from any fs event > > like write back, it fully ignore things like page_mkclean > > > > This is how it is to day, we allowed people to push upstream such > > users of GUP. This is a fact we have to live with, we can not stop > > hardware access to the page, we can not force the hardware to follow > > page_mkclean and force a page_mkwrite once write back ends. This is > > the situation we are inheriting (and i am personnaly not happy with > > that). > > > > >From my point of view we are left with 2 choices: > > [C1] break all drivers that do not abide by the page_mkclean and > > page_mkwrite > > [C2] mitigate as much as possible the issue > > > > For [C2] the idea is to keep track of GUP per page so we know if we > > can expect the page to be written to at any time. Here is the event > > flow: > > - driver GUP the page and program the hardware, page is mark as > > GUPed > > ... > > - write back kicks in on the dirty page, lock the page and every > > thing as usual , sees it is GUPed and inform the block layer to > > use a bounce page > > No. The solution John, Dan & I have been looking at is to take the > dirty page off the LRU while it is pinned by GUP. It will never be > found for writeback. > > That's not the end of the story though. Other parts of the kernel (eg > msync) also need to be taught to stay away from pages which are pinned > by GUP. But the idea is that no page gets written back to storage while > it's pinned by GUP. Only when the last GUP ends is the page returned > to the list of dirty pages. > > > - block layer copy the page to a bounce page effectively creating > > a snapshot of what is the content of the real page. This allows > > everything in block layer that need stable content to work on > > the bounce page (raid, stripping, encryption, ...) > > - once write back is done the page is not marked clean but stays > > dirty, this effectively disable things like COW for filesystem > > and other feature that expect page_mkwrite between write back. > > AFAIK it is believe that it is something acceptable > > So none of this is necessary. With the solution you are proposing we loose GUP fast and we have to allocate a structure for each page that is under GUP, and the LRU changes too. Moreover by not writing back there is a greater chance of data loss. I will do patches with the mapcount solution that require very little change and people will be able to choose which solution they prefer. Personaly i prefer the mapcount solution as it is less invasive. Cheers, Jérôme
