linux-kernel - Re: [PATCH v3 2/4] iov_iter: Add a function to extract a page list from an iterator

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Date:   Tue, 6 Dec 2022 11:40:32 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Christoph Hellwig <hch@...radead.org>,
        Jeff Layton <jlayton@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/4] iov_iter: Add a function to extract a page list
 from an iterator



On 2022-12-06 11:35, David Howells wrote:
> Logan Gunthorpe <logang@...tatee.com> wrote:
> 
>> If this is going to be a general replacement for iov_iter_get_pages()
>> it's going to need to pass through gup_flags. My recent patchset added
>> versions with these and I think it should be in during the next merge
>> cycle. [1]
> 
> Cool.  Note that the current iov_iter_get_pages2() is broken, which is why Al
> wanted a replacement.  It should not be taking a ref on the pages in an
> XARRAY, BVEC or PIPE - and it should be pinning rather than getting a ref on
> pages in IOVEC or UBUF if the buffer is being read into.  I'm guessing that
> your changes move the latter decision to the caller?

My changes maintained the status quo in terms of brokenness. They simply
added the gup_flags so I could pass a P2PDMA flag in a couple specific
cases. I have no objections to the other changes and vaguely look ok to
me, but having not seen patches to convert the users I care about, I
thought I'd point out that the P2PDMA use case will need to be supported
somehow.

Logan