linux-kernel - Re: [PATCH v6 03/34] iov_iter: Pass I/O direction into iov_iter_get

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Message-ID: <137f9567-411e-b411-3945-e553a57f6793@redhat.com>
Date:   Tue, 17 Jan 2023 09:47:58 +0100
From:   David Hildenbrand <david@...hat.com>
To:     David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v6 03/34] iov_iter: Pass I/O direction into
 iov_iter_get_pages*()

On 17.01.23 09:44, David Howells wrote:
> Christoph Hellwig <hch@...radead.org> wrote:
> 
>> On Mon, Jan 16, 2023 at 11:08:24PM +0000, David Howells wrote:
>>> Define FOLL_SOURCE_BUF and FOLL_DEST_BUF to indicate to get_user_pages*()
>>> and iov_iter_get_pages*() how the buffer is intended to be used in an I/O
>>> operation.  Don't use READ and WRITE as a read I/O writes to memory and
>>> vice versa - which causes confusion.
>>>
>>> The direction is checked against the iterator's data_source.
>>
>> Why can't we use the existing FOLL_WRITE?
> 
> Because FOLL_WRITE doesn't mean the same as WRITE:
> 
>   (1) It looks like it should really be FOLL_CHECK_PTES_WRITABLE.  It's not
>       defined as being anything to do with the I/O.

Especially combined with FOLL_FORCE, this is not true.

-- 
Thanks,

David / dhildenb