linux-kernel - Re: [PATCH] vmalloc: respect the GFP_NOIO and GFP

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Date:   Fri, 30 Jun 2017 20:36:12 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andreas Dilger <adilger@...ger.ca>,
        John Hubbard <jhubbard@...dia.com>,
        David Miller <davem@...emloft.net>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH] vmalloc: respect the GFP_NOIO and GFP_NOFS flags



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?

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.

Mikulas