On Fri 30-06-17 20:36:12, Mikulas Patocka wrote:

On Fri, 30 Jun 2017, Michal Hocko wrote:

quoted

On Fri 30-06-17 14:11:57, Mikulas Patocka wrote:

quoted

On Fri, 30 Jun 2017, Michal Hocko wrote:

quoted

On Thu 29-06-17 22:25:09, Mikulas Patocka wrote:

quoted

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

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