linux-ext4 - Re: [PATCH v2 05/18] xfs: Add xfs_break_layouts() to the inode eviction path

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Message-ID: <YyuoE8BgImRXVkkO@nvidia.com>
Date:   Wed, 21 Sep 2022 21:10:59 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Dave Chinner <david@...morbit.com>, akpm@...ux-foundation.org,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        linux-fsdevel@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 05/18] xfs: Add xfs_break_layouts() to the inode
 eviction path

On Wed, Sep 21, 2022 at 05:02:37PM -0700, Dan Williams wrote:

> The scenario I cannot convince myself is impossible is a driver that
> goes into interruptible sleep while operating on a page it got from
> get_user_pages(). Where the eventual driver completion path will clean
> up the pinned page, but the process that launched the I/O has already
> exited and dropped all the inode references it was holding. That's not
> buggy on its face since the driver still cleans up everything it was
> handed, but if this type of disconnect happens (closing mappings and
> files while I/O is in-flight) then iput_final() needs to check.

I don't think you can make this argument. The inode you are talking
about is held in the vma of the mm_struct, it is not just a process
exit or interrupted sleep that could cause the vma to drop the inode
reference, but any concurrent thread doing memunmap/close can destroy
the VMA, close the FD and release the inode.

So userspace can certainly create races where something has safely
done GUP/PUP !FOLL_LONGTERM but the VMA that sourced the page is
destroyed while the thread is still processing the post-GUP work.

Jason