linux-kernel - Re: [PATCH 01/10] cramfs: Convert to use vmf_insert

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Message-ID: <nycvar.YSQ.7.76.1808281940360.10215@knanqh.ubzr>
Date:   Tue, 28 Aug 2018 19:52:03 -0400 (EDT)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Souptick Joarder <jrdr.linux@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/10] cramfs: Convert to use vmf_insert_mixed

On Tue, 28 Aug 2018, Matthew Wilcox wrote:

> On Tue, Aug 28, 2018 at 01:49:25PM -0400, Nicolas Pitre wrote:
> > On Tue, 28 Aug 2018, Matthew Wilcox wrote:
> > > -			ret = vm_insert_mixed(vma, vma->vm_start + off, pfn);
> > > +			vmf = vmf_insert_mixed(vma, vma->vm_start + off, pfn);
> > > +			if (vmf & VM_FAULT_ERROR) {
> > > +				pages = i;
> > > +				break;
> > > +			}
> > 
> > I'd suggest this to properly deal with errers instead:
> > 
> > -			ret = vm_insert_mixed(vma, vma->vm_start + off, pfn);
> > +			vmf = vmf_insert_mixed(vma, vma->vm_start + off, pfn);
> > +			if (vmf & VM_FAULT_ERROR)
> > +				ret = vm_fault_to_errno(vmf, 0);
> 
> By my reading of this function, the intent is actually to return 0
> here and allow demand paging to work.  Of course, I've spent all of
> twenty minutes staring at this function, so I defer to the maintainer.

Demand paging is used when the filesystem layout isn't amenable to a 
direct mapping.  It is not a fallback for when we're OOM or some other 
internal errors which ought to be reported immediately.

> I think you'd need to be running a make-memory-allocations-fail fuzzer
> to hit this, so it's likely never been tested.

Well, it has been tested sort of, e.g. when vm_insert_mixed() returned 
an error due to misaligned addresses during development.  Normally, 
vm_insert_mixed() and vmf_insert_mixed() should always succeed, and if 
they don't we certainly don't want to ignore it.


Nicolas