linux-kernel - Re: [RFC] nvfs: a filesystem for persistent memory

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Message-ID: <20d31308-e60a-99f2-3309-c9f6c115e32b@linux.ibm.com>
Date:   Tue, 22 Sep 2020 15:34:37 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Mikulas Patocka <mpatocka@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Eric Sandeen <esandeen@...hat.com>,
        Dave Chinner <dchinner@...hat.com>,
        "Kani, Toshi" <toshi.kani@....com>,
        "Norton, Scott J" <scott.norton@....com>,
        "Tadakamadla, Rajesh (DCIG/CDI/HPS Perf)" 
        <rajesh.tadakamadla@....com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-nvdimm@...ts.01.org
Subject: Re: [RFC] nvfs: a filesystem for persistent memory



On 9/15/20 6:30 PM, Matthew Wilcox wrote:
> On Tue, Sep 15, 2020 at 08:34:41AM -0400, Mikulas Patocka wrote:
>> - when the fsck.nvfs tool mmaps the device /dev/pmem0, the kernel uses
>> buffer cache for the mapping. The buffer cache slows does fsck by a factor
>> of 5 to 10. Could it be possible to change the kernel so that it maps DAX
>> based block devices directly?
> 
> Oh, because fs/block_dev.c has:
>          .mmap           = generic_file_mmap,
> 
> I don't see why we shouldn't have a blkdev_mmap modelled after
> ext2_file_mmap() with the corresponding blkdev_dax_vm_ops.
> 

pls help with below 2 queries:-

1. Can't we use ->direct_IO here to avoid the mentioned performance problem?
2. Any other existing use case where having this blkdev_dax_vm_ops be 
useful?

-ritesh