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SubjectRe: [PATCH v2 07/17] fs: new infrastructure for writeback error handling and reporting
On Wed, Apr 12 2017, Jeff Layton wrote:


> +void __filemap_set_wb_error(struct address_space *mapping, int err)

I was really hoping that this would be

void __set_wb_error(wb_err_t *wb_err, int err)

so

Then nfs_context_set_write_error could become

static void nfs_context_set_write_error(struct nfs_open_context *ctx, int error)
{
__set_wb_error(&ctx->wb_err, error);
}

and filemap_set_sb_error() would be:

static inline void filemap_set_wb_error(struct address_space *mapping, int err)
{
/* Optimize for the common case of no error */
if (unlikely(err))
__set_wb_error(&mapping->f_wb_err, err);
}

Similarly we would have
wb_err_t sample_wb_error(wb_err_t *wb_err)
{
...
}

and

wb_err_t filemap_sample_wb_error(struct address_space *mapping)
{
return sample_wb_error(&mapping->f_wb_err);
}

so nfs_file_fsync_commit() could have
ret = sample_wb_error(&ctx->wb_err);
in place of
ret = xchg(&ctx->error, 0);

int filemap_report_wb_error(struct file *file)

would become

int filemap_report_wb_error(struct file *file, wb_err_t *err)

or something.

The address space is just one (obvious) place where the wb error can be
stored. The filesystem might have a different place with finer
granularity (nfs already does).


> +wb_err_t filemap_sample_wb_error(struct address_space *mapping)
> +{
> + wb_err_t old = READ_ONCE(mapping->wb_err);
> + wb_err_t new = old;
> +
> + /*
> + * For the common case of no errors ever having been set, we can skip
> + * marking the SEEN bit. Once an error has been set, the value will
> + * never go back to zero.
> + */
> + if (old != 0) {
> + new |= WB_ERR_SEEN;
> + if (old != new)
> + cmpxchg(&mapping->wb_err, old, new);
> + }
> + return new;
> +}

I do like how the use of cmpxchg work out here - no looping!

Thanks
NeilBrown
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