On Fri 30-06-17 20:36:12, Mikulas Patocka wrote: > > > On Fri, 30 Jun 2017, Michal Hocko wrote: > > > On Fri 30-06-17 14:11:57, Mikulas Patocka wrote: > > > > > > > > > On Fri, 30 Jun 2017, Michal Hocko wrote: > > > > > > > On Thu 29-06-17 22:25:09, Mikulas Patocka wrote: > > > > > The __vmalloc function has a parameter gfp_mask with the allocation > > > > > flags, > > > > > however it doesn't fully respect the GFP_NOIO and GFP_NOFS flags. The > > > > > pages are allocated with the specified gfp flags, but the pagetables > > > > > are > > > > > always allocated with GFP_KERNEL. This allocation can cause unexpected > > > > > recursion into the filesystem or I/O subsystem. > > > > > > > > > > It is not practical to extend page table allocation routines with gfp > > > > > flags because it would require modification of architecture-specific > > > > > code > > > > > in all architecturs. However, the process can temporarily request > > > > > that all > > > > > allocations are done with GFP_NOFS or GFP_NOIO with with the functions > > > > > memalloc_nofs_save and memalloc_noio_save. > > > > > > > > > > This patch makes the vmalloc code use memalloc_nofs_save or > > > > > memalloc_noio_save if the supplied gfp flags do not contain __GFP_FS > > > > > or > > > > > __GFP_IO. It fixes some possible deadlocks in drivers/mtd/ubi/io.c, > > > > > fs/gfs2/, fs/btrfs/free-space-tree.c, fs/ubifs/, > > > > > fs/nfs/blocklayout/extent_tree.c where __vmalloc is used with the > > > > > GFP_NOFS > > > > > flag. > > > > > > > > I strongly believe this is a step in the _wrong_ direction. Why? Because > > > > > > What do you think __vmalloc with GFP_NOIO should do? Print a warning? > > > Silently ignore the GFP_NOIO flag? > > > > I think noio users are not that much different from nofs users. Simply > > use the scope API at the place where the scope starts and document why > > it is needed. vmalloc calls do not have to be any special then and they > > do not even have to think about proper gfp flags and they can use > > whatever is the default. > > -- > > Michal Hocko > > SUSE Labs > > But you didn't answer the question - what should __vmalloc with GFP_NOIO > (or GFP_NOFS) do? Silently drop the flag? Print a warning? Or respect the > flag?
We can add a warning (or move it from kvmalloc) and hope that the respective maintainers will fix those places properly. The reason I didn't add the warning to vmalloc and kept it in kvmalloc was to catch only new users rather than suddenly splat on existing ones. Note that there are users with panic_on_warn enabled. Considering how many NOFS users we have in tree I would rather work with maintainers to fix them. > Currently, it silently drops the GFP_NOIO or GFP_NOFS flag, but some > programmers don't know it and use these flags. You can't blame those > programmers for not knowing it. At least __vmalloc_node is documented to not support all gfp flags. -- Michal Hocko SUSE Labs
