linux-ext4 - Re: [PATCH] mm: replace FAULT_FLAG_SIZE with parameter to huge

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Message-ID: <CAPcyv4jHYR2-_SgD7a6ab5vWigYsDoSb7FZdTchP8Xg+BF-2yg@mail.gmail.com>
Date:   Mon, 6 Feb 2017 08:24:48 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Dave Jiang <dave.jiang@...el.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-xfs@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jan Kara <jack@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm: replace FAULT_FLAG_SIZE with parameter to huge_fault

On Mon, Feb 6, 2017 at 6:36 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Fri, Feb 03, 2017 at 02:31:22PM -0700, Dave Jiang wrote:
>> Since the introduction of FAULT_FLAG_SIZE to the vm_fault flag, it has
>> been somewhat painful with getting the flags set and removed at the
>> correct locations. More than one kernel oops was introduced due to
>> difficulties of getting the placement correctly. Removing the flag
>> values and introducing an input parameter to huge_fault that indicates
>> the size of the page entry. This makes the code easier to trace and
>> should avoid the issues we see with the fault flags where removal of the
>> flag was necessary in the fallback paths.
>
> Why is this not in struct vm_fault?

Because this is easier to read and harder to get wrong. Same arguments
as getting rid of struct blk_dax_ctl.

> Also can be use this opportunity
> to fold ->huge_fault into ->fault?

Hmm, yes, just need a scheme to not attempt huge_faults on pte-only handlers.