linux-kernel - Re: [PATCH v3 02/13] dax: require 'struct page' for filesystem dax

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Date:   Fri, 20 Oct 2017 08:23:02 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Jeff Moyer <jmoyer@...hat.com>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>
Subject: Re: [PATCH v3 02/13] dax: require 'struct page' for filesystem dax

On Fri, Oct 20, 2017 at 12:57 AM, Christoph Hellwig <hch@....de> wrote:
>> --- a/arch/powerpc/sysdev/axonram.c
>> +++ b/arch/powerpc/sysdev/axonram.c
>> @@ -172,6 +172,7 @@ static size_t axon_ram_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff,
>>
>>  static const struct dax_operations axon_ram_dax_ops = {
>>       .direct_access = axon_ram_dax_direct_access,
>> +
>>       .copy_from_iter = axon_ram_copy_from_iter,
>
> Unrelated whitespace change.  That being said - I don't think axonram has
> devmap support in any form, so this basically becomes dead code, doesn't
> it?
>
>> diff --git a/drivers/s390/block/dcssblk.c b/drivers/s390/block/dcssblk.c
>> index 7abb240847c0..e7e5db07e339 100644
>> --- a/drivers/s390/block/dcssblk.c
>> +++ b/drivers/s390/block/dcssblk.c
>> @@ -52,6 +52,7 @@ static size_t dcssblk_dax_copy_from_iter(struct dax_device *dax_dev,
>>
>>  static const struct dax_operations dcssblk_dax_ops = {
>>       .direct_access = dcssblk_dax_direct_access,
>> +
>>       .copy_from_iter = dcssblk_dax_copy_from_iter,
>
> Same comments apply here.

Yes, however it seems these drivers / platforms have been living with
the lack of struct page for a long time. So they either don't use DAX,
or they have a constrained use case that never triggers
get_user_pages(). If it is the latter then they could introduce a new
configuration option that bypasses the pfn_t_devmap() check in
bdev_dax_supported() and fix up the get_user_pages() paths to fail.
So, I'd like to understand how these drivers have been using DAX
support without struct page to see if we need a workaround or we can
go ahead delete this support. If the usage is limited to
execute-in-place perhaps we can do a constrained ->direct_access() for
just that case.