Em Tue, 7 Nov 2017 09:43:41 -0800 Dan Williams <dan.j.williams@xxxxxxxxx> escreveu: > On Tue, Nov 7, 2017 at 12:33 AM, Mauro Carvalho Chehab > <mchehab@xxxxxxxxxxxxxxxx> wrote: > > Em Mon, 06 Nov 2017 16:57:28 -0800 > > Dan Williams <dan.j.williams@xxxxxxxxx> escreveu: > > > >> V4L2 memory registrations are incompatible with filesystem-dax that > >> needs the ability to revoke dma access to a mapping at will, or > >> otherwise allow the kernel to wait for completion of DMA. The > >> filesystem-dax implementation breaks the traditional solution of > >> truncate of active file backed mappings since there is no page-cache > >> page we can orphan to sustain ongoing DMA. > >> > >> If v4l2 wants to support long lived DMA mappings it needs to arrange to > >> hold a file lease or use some other mechanism so that the kernel can > >> coordinate revoking DMA access when the filesystem needs to truncate > >> mappings. > > > > > > Not sure if I understand this your comment here... what happens > > if FS_DAX is enabled? The new err = get_user_pages_longterm() > > would cause DMA allocation to fail? > > Correct, any attempt to specify a filesystem-dax mapping range to > get_user_pages_longterm will fail with EOPNOTSUPP. In the future we > want to add something like a 'struct file_lock *' argument to > get_user_pages_longterm so that the kernel has a handle to revoke > access to the returned pages. Once we have a safe way for the kernel > to undo elevated page counts we can stop failing the longterm vs > filesystem-dax case. Argh! Perhaps we should make it depend on BROKEN while not fixed :-/ > Here is more background on why _longterm gup is a problem for filesystem-dax: > > https://lwn.net/Articles/737273/ > > > If so, that doesn't sound > > right. Instead, mm should somehow mark this mapping to be out > > of FS_DAX control range. > > DAX is currently global setting for the entire backing device of the > filesystem, so any mapping of any file when the "-o dax" mount option > is set is in the "FS_DAX control range". In other words there's > currently no way to prevent FS_DAX mappings from being exposed to V4L2 > outside of CONFIG_FS_DAX=n. Grrr... > > Also, it is not only videobuf-dma-sg.c that does long lived > > DMA mappings. VB2 also does that (and videobuf-vmalloc). > > Without finding the code videobuf-vmalloc sounds like it should be ok > if the kernel is allocating memory separate from a file-backed DAX > mapping. videobuf-vmalloc do DMA mapping for pages allocated via vmalloc(), via vmalloc_user()/remap_vmalloc_range(). There aren't much drivers using VB1 anymore, but a change at VB2 will likely break support for almost all webcams if fs DAX is in usage. > Where is the VB2 get_user_pages call? Before changeset 3336c24f25ec, the logic for get_user_pages() were at drivers/media/v4l2-core/videobuf2-dma-sg.c. Now, the logic it uses is inside mm/frame_vector.c. Thanks, Mauro
