On 2/18/21 5:39 AM, Matthew Wilcox wrote:
On Thu, Feb 18, 2021 at 08:42:14PM +0800, Qu Wenruo wrote:
[...]
BTW, what would be the extra cost by converting page::index to u64?
I know tons of printk() would cause warning, but most 64bit systems
should not be affected anyway.
No effect for 64-bit systems, other than the churn.
For 32-bit systems, it'd have some pretty horrible overhead. You don't
just have to touch the page cache, you have to convert the XArray.
It's doable (I mean, it's been done), but it's very costly for all the
32-bit systems which don't use a humongous filesystem. And we could
minimise that overhead with a typedef, but then the source code gets
harder to work with.
Out of curiosity, would it be at all feasible to use 64-bits for the
page offset *without* changing XArray, perhaps by indexing by the lower
32-bits, and evicting the page that's there if the top bits don't match
(vaguely like how the CPU cache works)? Or, if there are cases where a
page can't be evicted (I don't know if this can ever happen), use chaining?
I would expect index contention to be extremely uncommon, and it could
only happen for inodes larger than 16 TiB, which can't be used at all
today. I don't know how many data structures store page offsets today,
but it seems like this should significantly reduce the performance
impact versus upping XArray to 64-bit indexes.
