by Christoph Hellwig

[permalink] [raw]

Subject: Re: [PATCH v6 4/6] dax: add DAX_RECOVERY flag and .recovery_write dev_pgmap_ops

On Tue, Mar 22, 2022 at 11:05:09PM +0000, Jane Chu wrote:
> > This DAX_RECOVERY doesn't actually seem to be used anywhere here or
> > in the subsequent patches. Did I miss something?
>
> dax_iomap_iter() uses the flag in the same patch,
> + if ((map_len == -EIO) && (iov_iter_rw(iter) == WRITE)) {
> + flags |= DAX_RECOVERY;
> + map_len = dax_direct_access(dax_dev, pgoff, nrpg,
> + flags, &kaddr, NULL);

Yes, it passes it on to dax_direct_access, and dax_direct_access passes
it onto ->direct_access. But nothing in this series actually checks
for it as far as I can tell.

> >> Also introduce a new dev_pagemap_ops .recovery_write function.
> >> The function is applicable to FSDAX device only. The device
> >> page backend driver provides .recovery_write function if the
> >> device has underlying mechanism to clear the uncorrectable
> >> errors on the fly.
> >
> > Why is this not in struct dax_operations?
>
> Per Dan's comments to the v5 series, adding .recovery_write to
> dax_operations causes a number of trivial dm targets changes.
> Dan suggested that adding .recovery_write to pagemap_ops could
> cut short the logistics of figuring out whether the driver backing
> up a page is indeed capable of clearing poison. Please see
> https://lkml.org/lkml/2022/2/4/31

But at least in this series there is 1:1 association between the
pgmap and the dax_device so that scheme won't work. It would
have to lookup the pgmap based on the return physical address from
dax_direct_access. Which sounds more complicated than just adding
the (annoying) boilerplate code to DM.

> include/linux/memremap.h doesn't know struct iov_iter which is defined
> in include/linux/uio.h, would you prefer to adding include/linux/uio.h
> to include/linux/memremap.h ?

As it is not derefences just adding a

struct iov_iter;

line to memremap.h below the includes should be all that is needed.

2022-03-26 10:27:55

by Jane Chu

[permalink] [raw]

Subject: Re: [PATCH v6 4/6] dax: add DAX_RECOVERY flag and .recovery_write dev_pgmap_ops

On 3/22/2022 10:45 PM, Christoph Hellwig wrote:
> On Tue, Mar 22, 2022 at 11:05:09PM +0000, Jane Chu wrote:
>>> This DAX_RECOVERY doesn't actually seem to be used anywhere here or
>>> in the subsequent patches. Did I miss something?
>>
>> dax_iomap_iter() uses the flag in the same patch,
>> + if ((map_len == -EIO) && (iov_iter_rw(iter) == WRITE)) {
>> + flags |= DAX_RECOVERY;
>> + map_len = dax_direct_access(dax_dev, pgoff, nrpg,
>> + flags, &kaddr, NULL);
>
> Yes, it passes it on to dax_direct_access, and dax_direct_access passes
> it onto ->direct_access. But nothing in this series actually checks
> for it as far as I can tell.
>
>>>> Also introduce a new dev_pagemap_ops .recovery_write function.
>>>> The function is applicable to FSDAX device only. The device
>>>> page backend driver provides .recovery_write function if the
>>>> device has underlying mechanism to clear the uncorrectable
>>>> errors on the fly.
>>>
>>> Why is this not in struct dax_operations?
>>
>> Per Dan's comments to the v5 series, adding .recovery_write to
>> dax_operations causes a number of trivial dm targets changes.
>> Dan suggested that adding .recovery_write to pagemap_ops could
>> cut short the logistics of figuring out whether the driver backing
>> up a page is indeed capable of clearing poison. Please see
>> https://lkml.org/lkml/2022/2/4/31
>
> But at least in this series there is 1:1 association between the
> pgmap and the dax_device so that scheme won't work. It would
> have to lookup the pgmap based on the return physical address from
> dax_direct_access. Which sounds more complicated than just adding
> the (annoying) boilerplate code to DM.
>

Getting dax_direct_access to return pfn seems straight forward,
what do you think of below change?

--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -195,10 +195,10 @@ int dax_zero_page_range(struct dax_device
*dax_dev, pgoff_t pgoff,
}
EXPORT_SYMBOL_GPL(dax_zero_page_range);

-size_t dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
+size_t dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
pfn_t pfn,
void *addr, size_t bytes, struct iov_iter *iter)
{
- struct dev_pagemap *pgmap = dax_dev->pgmap;
+ struct dev_pagemap *pgmap = get_dev_pagemap(pfn_t_to_pfn(pfn),
NULL);

if (!pgmap || !pgmap->ops->recovery_write)
return -EIO;

--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1243,6 +1243,7 @@ static loff_t dax_iomap_iter(const struct
iomap_iter *iomi,
int flags, recov;
void *kaddr;
long nrpg;
+ pfn_t pfn;

if (fatal_signal_pending(current)) {
ret = -EINTR;
@@ -1257,7 +1258,7 @@ static loff_t dax_iomap_iter(const struct
iomap_iter *iomi,
if ((map_len == -EIO) && (iov_iter_rw(iter) == WRITE)) {
flags |= DAX_RECOVERY;
map_len = dax_direct_access(dax_dev, pgoff, nrpg,
- flags, &kaddr, NULL);
+ flags, &kaddr, &pfn);
if (map_len > 0)
recov++;
}
@@ -1273,7 +1274,7 @@ static loff_t dax_iomap_iter(const struct
iomap_iter *iomi,
map_len = end - pos;

if (recov)
- xfer = dax_recovery_write(dax_dev, pgoff, kaddr,
+ xfer = dax_recovery_write(dax_dev, pgoff, pfn,
kaddr,
map_len, iter);
else if (iov_iter_rw(iter) == WRITE)
xfer = dax_copy_from_iter(dax_dev, pgoff, kaddr,

thanks!
-jane