On 05/02/23 20:05, Mike Kravetz wrote:
> On 05/02/23 23:56, Ackerley Tng wrote:
> > When fallocate() is called twice on the same offset in the file, the
> > second fallocate() should succeed.
> >
> > page_cache_next_miss() always advances index before returning, so even
> > on a page cache hit, the check would set present to false.
>
> Thank you Ackerley for finding this!
>
> When I read the description of page_cache_next_miss(), I assumed
>
> present = page_cache_next_miss(mapping, index, 1) != index;
>
> would tell us if there was a page at index in the cache.
>
> However, when looking closer at the code it does not check for a page
> at index, but rather starts looking at index+1. Perhaps that is why
> it is named next?
>
> Matthew, I think the use of the above statement was your suggestion.
> And you know the xarray code better than anyone. I just want to make
> sure page_cache_next_miss is operating as designed/expected. If so,
> then the changes suggested here make sense.
I took a closer look at the code today.
page_cache_next_miss has a 'special case' for index 0. The function
description says:
* Return: The index of the gap if found, otherwise an index outside the
* range specified (in which case 'return - index >= max_scan' will be true).
* In the rare case of index wrap-around, 0 will be returned.
And, the loop in the routine does:
while (max_scan--) {
void *entry = xas_next(&xas);
if (!entry || xa_is_value(entry))
break;
if (xas.xa_index == 0)
break;
}
At first glance, I thought xas_next always went to the next entry but
now see that is not the case here because this is a new state with
xa_node = XAS_RESTART. So, xas_next is effectively a xas_load.
This means in the case were index == 0,
page_cache_next_miss(mapping, index, 1)
will ALWAYS return zero even if a page is present.
I need to look at the xarray code and this rare index wrap-around case
to see if we can somehow modify that check for xas.xa_index == 0 in
page_cache_next_miss.
--
Mike Kravetz
