On Thu, Apr 06, 2017 at 03:14:52PM -0400, Jeff Layton wrote:
> @@ -868,6 +869,7 @@ struct file {
> struct list_head f_tfile_llink;
> #endif /* #ifdef CONFIG_EPOLL */
> struct address_space *f_mapping;
> + u32 f_wb_err;
> } __attribute__((aligned(4))); /* lest something weird decides that 2 is OK */
>
I think we can squeeze that in next to f_flags?
> +/**
> + * filemap_set_wb_error - set the wb error in the mapping for later reporting
> + * @mapping: mapping in which the error should be set
> + * @err: error to set. must be negative value but not less than -MAX_ERRNO
Do we want to have users call filemap_set_wb_error(mapping, EIO)
or filemap_set_wb_error(mapping, -EIO)? Either way, we can assert
that it's in the correct range (oh look, we have at least one user of
mapping_set_error calling it with a positive errno ...)
I've been playing with positive or negative errnos for the xarray, and
positive looks better to me, although there's a definite advantage to
being able to just call filemap_set_wb_error(mapping, result).
#define XAS_ERROR(errno) ((struct xa_node *)((errno << 1) | 1))
static inline int xas_error(const struct xa_state *xas)
{
unsigned long v = (unsigned long)xas->xa_node;
return (v & 1) ? -(v >> 1) : 0;
}
static inline void xas_set_err(struct xa_state *xas, unsigned long err)
{
XA_BUG_ON(err > MAX_ERRNO);
xas->xa_node = XAS_ERROR(err);
}
> + /*
> + * Ensure the error code actually fits where we want it to go. If it
> + * doesn't then just throw a warning and don't record anything.
> + */
> + if (unlikely(err > 0 || err < -MAX_ERRNO)) {
> + WARN(1, "err=%d\n", err);
> + return;
> + }
Cute trick to make this more succinct:
if (WARN(err > 0 || err < -MAX_ERRNO), "err = %d\n", err)
return;
or even ...
if (WARN((unsigned int)-err > MAX_ERRNO), "err = %d\n", err)
return;
> + /* Clear out error bits and set new error */
> + new = (old & ~MAX_ERRNO) | -err;
> +
> + /* Only increment if someone has looked at it */
> + if (old & WB_ERR_SEEN) {
> + new += WB_ERR_CTR_INC;
> + new &= ~WB_ERR_SEEN;
> + }
Although we always want to clear out the SEEN bit if we're updating ... so
new = (old & ~(MAX_ERRNO | WB_ERR_SEEN) | -err;
/* Only increment if someone has looked at it */
if (old & WB_ERR_SEEN)
new += WB_ERR_CTR_INC;
... and then there's no need to update if it's the same errno and nobody's
seen it:
if (old == new)
break;
[...]
> + /*
> + * We always store values with the "seen" bit set, so if this
> + * matches what we already have, then we can call it done.
> + * There is nothing to update so just return 0.
> + */
> + if (old == file->f_wb_err)
> + break;
> +
> + /* set flag and try to swap it into place */
> + new = old | WB_ERR_SEEN;
Again, I think we should avoid the cmpxchg with:
if (old == new)
break;
> + cur = cmpxchg(&mapping->wb_err, old, new);
> +
> + /*
> + * We can quit now if we successfully swapped in the new value
> + * or someone else beat us to it with the same value that we
> + * were planning to store.
> + */
> + if (likely(cur == old || cur == new)) {
> + file->f_wb_err = new;
> + err = -(new & MAX_ERRNO);
> + break;
> + }
> +
> + /* Raced with an update, try again */
> + old = cur;
Well ... should we? We're returning an error which is new to this fd anyway.
Do we want to return the most recent error by a nanosecond, or should we
return the previous one and then see this one next time we call fsync()?
I'd lean towards not looping here; not even looking at 'cur'.
