linux-kernel - Re: [PATCH 2/2] mm/hmm: hmm_range

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Message-ID: <20190824223907.GB21891@lst.de>
Date:   Sun, 25 Aug 2019 00:39:07 +0200
From:   Christoph Hellwig <hch@....de>
To:     Ralph Campbell <rcampbell@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        nouveau@...ts.freedesktop.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 2/2] mm/hmm: hmm_range_fault() infinite loop

On Fri, Aug 23, 2019 at 03:17:53PM -0700, Ralph Campbell wrote:
> Normally, callers to handle_mm_fault() are supposed to check the
> vma->vm_flags first. hmm_range_fault() checks for VM_READ but doesn't
> check for VM_WRITE if the caller requests a page to be faulted in
> with write permission (via the hmm_range.pfns[] value).
> If the vma is write protected, this can result in an infinite loop:
>   hmm_range_fault()
>     walk_page_range()
>       ...
>       hmm_vma_walk_hole()
>         hmm_vma_walk_hole_()
>           hmm_vma_do_fault()
>             handle_mm_fault(FAULT_FLAG_WRITE)
>             /* returns VM_FAULT_WRITE */
>           /* returns -EBUSY */
>         /* returns -EBUSY */
>       /* returns -EBUSY */
>     /* loops on -EBUSY and range->valid */
> Prevent this by checking for vma->vm_flags & VM_WRITE before calling
> handle_mm_fault().
> 
> Signed-off-by: Ralph Campbell <rcampbell@...dia.com>

Looks good,

Reviewed-by: Christoph Hellwig <hch@....de>