Re: [RFC PATCH v3 2/4] dax: Check for data cache aliasing at runtime

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Dan Williams <dan.j.williams@intel.com>,
	Arnd Bergmann <arnd@arndb.de>, Dave Chinner <david@fromorbit.com>
Cc: linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-mm@kvack.org, linux-arch@vger.kernel.org,
	Vishal Verma <vishal.l.verma@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	Matthew Wilcox <willy@infradead.org>,
	Russell King <linux@armlinux.org.uk>,
	nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, dm-devel@lists.linux.dev
Subject: Re: [RFC PATCH v3 2/4] dax: Check for data cache aliasing at runtime
Date: Fri, 2 Feb 2024 11:32:00 -0500	[thread overview]
Message-ID: <da4c7c2c-0400-40d7-8263-22d284ecca8c@efficios.com> (raw)
In-Reply-To: <f1d14941-2d22-452a-99e6-42db806b6d7f@efficios.com>

On 2024-02-01 10:44, Mathieu Desnoyers wrote:
> On 2024-01-31 17:18, Dan Williams wrote:

[...]

>> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
>> index 5f1be1da92ce..11053a70f5ab 100644
>> --- a/fs/fuse/virtio_fs.c
>> +++ b/fs/fuse/virtio_fs.c
>> @@ -16,6 +16,7 @@
>>   #include <linux/fs_context.h>
>>   #include <linux/fs_parser.h>
>>   #include <linux/highmem.h>
>> +#include <linux/cleanup.h>
>>   #include <linux/uio.h>
>>   #include "fuse_i.h"
>> @@ -795,8 +796,11 @@ static void virtio_fs_cleanup_dax(void *data)
>>       put_dax(dax_dev);
>>   }
>> +DEFINE_FREE(cleanup_dax, struct dax_dev *, if (!IS_ERR_OR_NULL(_T)) 
>> virtio_fs_cleanup_dax(_T))
>> +
>>   static int virtio_fs_setup_dax(struct virtio_device *vdev, struct 
>> virtio_fs *fs)
> 
> So either I'm completely missing how ownership works in this function, or
> we should be really concerned about the fact that it does no actual
> cleanup of anything on any error.
[...]
> 
> Here what I'm seeing so far:
> 
> - devm_release_mem_region() is never called after 
> devm_request_mem_region(). Not
>    on error, neither on teardown,
> - pgmap is never freed on error after devm_kzalloc.

I was indeed missing something: the devm_ family of functions
keeps ownership at the device level, so we would not need explicit
teardown.

> 
>>   {
>> +    struct dax_device *dax_dev __free(cleanup_dax) = NULL;
>>       struct virtio_shm_region cache_reg;
>>       struct dev_pagemap *pgmap;
>>       bool have_cache;
>> @@ -804,6 +808,15 @@ static int virtio_fs_setup_dax(struct 
>> virtio_device *vdev, struct virtio_fs *fs)
>>       if (!IS_ENABLED(CONFIG_FUSE_DAX))
>>           return 0;
>> +    dax_dev = alloc_dax(fs, &virtio_fs_dax_ops);
>> +    if (IS_ERR(dax_dev)) {
>> +        int rc = PTR_ERR(dax_dev);
>> +
>> +        if (rc == -EOPNOTSUPP)
>> +            return 0;
>> +        return rc;
>> +    }
> 
> What is gained by moving this allocation here ?

I'm still concerned about moving the call to alloc_dax() before
the setup of the memory region it will use. Are those completely
independent ?

> 
>> +
>>       /* Get cache region */
>>       have_cache = virtio_get_shm_region(vdev, &cache_reg,
>>                          (u8)VIRTIO_FS_SHMCAP_ID_CACHE);
>> @@ -849,10 +862,7 @@ static int virtio_fs_setup_dax(struct 
>> virtio_device *vdev, struct virtio_fs *fs)
>>       dev_dbg(&vdev->dev, "%s: window kaddr 0x%px phys_addr 0x%llx len 
>> 0x%llx\n",
>>           __func__, fs->window_kaddr, cache_reg.addr, cache_reg.len);
>> -    fs->dax_dev = alloc_dax(fs, &virtio_fs_dax_ops);
>> -    if (IS_ERR(fs->dax_dev))
>> -        return PTR_ERR(fs->dax_dev);
>> -
>> +    fs->dax_dev = no_free_ptr(dax_dev);
>>       return devm_add_action_or_reset(&vdev->dev, virtio_fs_cleanup_dax,
>>                       fs->dax_dev);
>>   }
> 

[...]

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com