linux-kernel - RE: [Patch v2 02/15] CIFS: Add support for direct pages in rdata

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Date:   Wed, 27 Jun 2018 03:21:53 +0000
From:   Long Li <longli@...rosoft.com>
To:     Tom Talpey <tom@...pey.com>, Jason Gunthorpe <jgg@...pe.ca>
CC:     Steve French <sfrench@...ba.org>,
        "linux-cifs@...r.kernel.org" <linux-cifs@...r.kernel.org>,
        "samba-technical@...ts.samba.org" <samba-technical@...ts.samba.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Subject: RE: [Patch v2 02/15] CIFS: Add support for direct pages in rdata

> Subject: Re: [Patch v2 02/15] CIFS: Add support for direct pages in rdata
> 
> On 6/25/2018 5:01 PM, Jason Gunthorpe wrote:
> > On Sat, Jun 23, 2018 at 09:50:20PM -0400, Tom Talpey wrote:
> >> On 5/30/2018 3:47 PM, Long Li wrote:
> >>> From: Long Li <longli@...rosoft.com>
> >>>
> >>> Add a function to allocate rdata without allocating pages for data
> >>> transfer. This gives the caller an option to pass a number of pages
> >>> that point to the data buffer.
> >>>
> >>> rdata is still reponsible for free those pages after it's done.
> >>
> >> "Caller" is still responsible? Or is the rdata somehow freeing itself
> >> via another mechanism?
> >>
> >>>
> >>> Signed-off-by: Long Li <longli@...rosoft.com>
> >>>   fs/cifs/cifsglob.h |  2 +-
> >>>   fs/cifs/file.c     | 23 ++++++++++++++++++++---
> >>>   2 files changed, 21 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/fs/cifs/cifsglob.h b/fs/cifs/cifsglob.h index
> >>> 8d16c3e..56864a87 100644
> >>> +++ b/fs/cifs/cifsglob.h
> >>> @@ -1179,7 +1179,7 @@ struct cifs_readdata {
> >>>   	unsigned int			tailsz;
> >>>   	unsigned int			credits;
> >>>   	unsigned int			nr_pages;
> >>> -	struct page			*pages[];
> >>> +	struct page			**pages;
> >>
> >> Technically speaking, these are syntactically equivalent. It may not
> >> be worth changing this historic definition.
> >
> > [] is a C99 'flex array', it has a different allocation behavior than
> > ** and is not interchangeable..
> 
> In that case, it's an even better reason to not change the declaration.

No, it needs to be declared separately.

With Direct I/O, **pages are allocated and returned from iov_iter_get_pages_alloc() when locking those user pages. They can't be allocated as part of struct cifs_readdata.