linux-kernel - Re: [PATCH 6/7] mm, fs: introduce file_operations->post

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Date:   Tue, 26 Sep 2017 14:41:53 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Christoph Hellwig <hch@....de>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...chiereds.net>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH 6/7] mm, fs: introduce file_operations->post_mmap()

On Tue, Sep 26, 2017 at 2:06 PM, Ross Zwisler
<ross.zwisler@...ux.intel.com> wrote:
> On Tue, Sep 26, 2017 at 12:19:21PM -0700, Dan Williams wrote:
>> On Tue, Sep 26, 2017 at 11:57 AM, Ross Zwisler
> <>
>> > This decision can only be made (in this
>> > proposed scheme) *after* the inode->i_mapping->i_mmap  tree has been
>> > populated, which means we need another call into the filesystem after this
>> > insertion has happened.
>>
>> I get that, but it seems over-engineered and something that can also
>> be safely cleaned up after the fact by the code path that is disabling
>> DAX.
>
> I don't think you can safely clean it up after the fact because some thread
> might have already called ->mmap() to set up the vma->vm_flags for their new
> mapping, but they haven't added it to inode->i_mapping->i_mmap.

If madvise(MADV_NOHUGEPAGE) can dynamically change vm_flags, then the
DAX disable path can as well. VM_MIXEDMAP looks to be a nop for normal
memory mappings.

> The inode->i_mapping->i_mmap tree is the only way (that I know of at least)
> that the filesystem has any idea about about the mapping.  This is the method
> by which we would try and clean up mapping flags, if we were to do so, and
> it's the only way that the filesystem can know whether or not mappings exist.
>
> The only way that I could think of to make this safely work is to have the
> insertion into the inode->i_mapping->i_mmap tree be our sync point.  After
> that the filesystem and the mapping code can communicate on the state of DAX,
> but before that I think it's basically indeterminate.

If we lose the race and leak VM_HUGEPAGE to a non-DAX mapping what
breaks? I'd rather be in favor of not setting VM_HUGEPAGE at all in
the ->mmap() handler and let the default THP policy take over. In
fact, see transparent_hugepage_enabled() we already auto-enable huge
page support for dax mappings regardless of VM_HUGEPAGE.