Re: [PATCH v10] mm: introduce MEMORY_DEVICE_FS_DAX and CONFIG_DEV_PAGEMAP_OPS

[Dan Williams <dan.j.williams@intel.com>]

On Fri, May 18, 2018 at 2:46 AM, Christoph Hellwig <hch@lst.de> wrote:
> This looks reasonable to me.  A few more comments below.
>
>> This patch replaces and consolidates patch 2 [1] and 4 [2] from the v9
>> series [3] for "dax: fix dma vs truncate/hole-punch".
>
> Can you repost the whole series?  Otherwise things might get a little
> too confusing.

Sure thing.

>>               WARN_ON(IS_ENABLED(CONFIG_ARCH_HAS_PMEM_API));
>> +             return 0;
>>       } else if (pfn_t_devmap(pfn)) {
>> +             struct dev_pagemap *pgmap;
>
> This should probably become something like:
>
>         bool supported = false;
>
>         ...
>
>
>         if (IS_ENABLED(CONFIG_FS_DAX_LIMITED) && pfn_t_special(pfn)) {
>                 ...
>                 supported = true;
>         } else if (pfn_t_devmap(pfn)) {
>                 pgmap = get_dev_pagemap(pfn_t_to_pfn(pfn), NULL);
>                 if (pgmap && pgmap->type == MEMORY_DEVICE_FS_DAX)
>                         supported = true;
>                 put_dev_pagemap(pgmap);
>         }
>
>         if (!supported) {
>                 pr_debug("VFS (%s): error: dax support not enabled\n",
>                         sb->s_id);
>                 return -EOPNOTSUPP;
>         }
>         return 0;

Looks good, will do.

>> +     select DEV_PAGEMAP_OPS if (ZONE_DEVICE && !FS_DAX_LIMITED)
>
> Btw, what was the reason again we couldn't get rid of FS_DAX_LIMITED?

The last I heard from Gerald they were still mildly interested in
keeping the dccssblk dax support going with this limited mode, and
threatened to add full page support at a later date:

---
From: Gerald

dcssblk seems to work fine, I did not see any SIGBUS while "executing
in place" from dcssblk with the current upstream kernel, maybe because
we only use dcssblk with fs dax in read-only mode.

Anyway, the dcssblk change is fine with me. I will look into adding
struct pages for dcssblk memory later, to make it work again with
this change, but for now I do not know of anyone needing this in the
upstream kernel.

https://www.spinics.net/lists/linux-xfs/msg14628.html
---

>> +void generic_dax_pagefree(struct page *page, void *data)
>> +{
>> +     wake_up_var(&page->_refcount);
>> +}
>> +EXPORT_SYMBOL_GPL(generic_dax_pagefree);
>
> Why is this here and exported instead of static in drivers/nvdimm/pmem.c?

I was thinking it did not belong to the pmem driver, but you're right
unless / until we grow another fsdax capable driver this detail can
stay internal to the pmem driver.