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Date:   Fri, 28 Jun 2019 09:44:50 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Damien Le Moal <Damien.LeMoal@wdc.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
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        Jens Axboe <axboe@kernel.dk>,
        Bart Van Assche <bvanassche@acm.org>
Subject: Re: [PATCH V5 2/3] sd_zbc: Fix report zones buffer allocation
Message-ID: <20190628074450.GA29550@lst.de>
References: <20190627092944.20957-1-damien.lemoal@wdc.com> <20190627092944.20957-3-damien.lemoal@wdc.com> <20190627140900.GB6209@lst.de> <BYAPR04MB581641F8E665ECD324B6C397E7FC0@BYAPR04MB5816.namprd04.prod.outlook.com>
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On Fri, Jun 28, 2019 at 07:30:49AM +0000, Damien Le Moal wrote:
> 
> Yes, indeed. However, removing the gfp_flags from report_zones method
> would limit possibilities to only GFP_NOIO or GFP_KERNEL (default
> vmalloc). What if the caller is an FS and needs GFP_NOFS, or any other
> reclaim flags ? Handling all possibilities does not seem reasonable.
> Handling only GFP_KERNEL and GFP_IO is easy, but that would mean that
> the caller of blkdev_report_zones would need to do itself calls to
> whatever  memalloc_noXX_save/restore() it needs. Is that OK ?

I think it is ok.  The only real possibily is noio anyway as far as
I can tell.

> 
> Currently, blkdev_report_zones() uses only either GFP_KERNEL (general
> case, fs, dm and user ioctl), or GFP_NOIO for revalidate, disk scan and
> dm-zoned error path. So removing the flag from the report zones method
> while keeping it in the block layer API to distinguished these cases is
> simple, but I am not sure if that will not pause problems for some
> users. Thoughts ?

I'd kill it from the block layer API and require the caller to set
the per-task flag.  If I understood the mm maintainers correctly the
long term plan is to kill of GFP_NOFS and GFP_NOIO flowly and just rely
on the contexts.