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

[Mathieu Desnoyers]

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