linux-kernel - Re: [PATCH v4 1/2] mm: refactor __vunmap() to avoid duplicated call to find_vm

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Message-Id: <20190418152431.c583ef892a8028c662db3e6a@linux-foundation.org>
Date:   Thu, 18 Apr 2019 15:24:31 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Roman Gushchin <guroan@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Johannes Weiner <hannes@...xchg.org>,
        Vlastimil Babka <vbabka@...e.cz>, Roman Gushchin <guro@...com>,
        Christoph Hellwig <hch@....de>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [PATCH v4 1/2] mm: refactor __vunmap() to avoid duplicated call
 to find_vm_area()

On Thu, 18 Apr 2019 04:18:34 -0700 Matthew Wilcox <willy@...radead.org> wrote:

> On Wed, Apr 17, 2019 at 02:58:27PM -0700, Andrew Morton wrote:
> > On Wed, 17 Apr 2019 12:40:01 -0700 Roman Gushchin <guroan@...il.com> wrote:
> > > +static struct vm_struct *__remove_vm_area(struct vmap_area *va)
> > > +{
> > > +	struct vm_struct *vm = va->vm;
> > > +
> > > +	might_sleep();
> > 
> > Where might __remove_vm_area() sleep?
> > 
> > >From a quick scan I'm only seeing vfree(), and that has the
> > might_sleep_if(!in_interrupt()).
> > 
> > So perhaps we can remove this...
> 
> See commit 5803ed292e63 ("mm: mark all calls into the vmalloc subsystem as potentially sleeping")
> 
> It looks like the intent is to unconditionally check might_sleep() at
> the entry points to the vmalloc code, rather than only catch them in
> the occasional place where it happens to go wrong.

afaict, vfree() will only do a mutex_trylock() in
try_purge_vmap_area_lazy().  So does vfree actually sleep in any
situation?  Whether or not local interrupts are enabled?