Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

[Matthew Wilcox]

On Mon, Sep 11, 2023 at 01:50:53PM -0700, Linus Torvalds wrote:
> Another example of this this is just plain read/write. It's not a
> problem in practice right now, because large pages are effectively
> never used.
> 
> But just imagine what happens once filemap_read() actually does big folios?
> 
> Do you really want this code:
> 
>         copied = copy_folio_to_iter(folio, offset, bytes, iter);
> 
> to forever use the artificial chunking it does now?
> 
> And yes, right now it will still do things in one-page chunks in
> copy_page_to_iter(). It doesn't even have cond_resched() - it's
> currently in the caller, in filemap_read().

Ah, um.  If you take a look in fs/iomap/buffered-io.c, you'll
see ...

iomap_write_iter:
        size_t chunk = PAGE_SIZE << MAX_PAGECACHE_ORDER;
                struct folio *folio;
                bytes = min(chunk - offset, iov_iter_count(i));
                if (unlikely(fault_in_iov_iter_readable(i, bytes) == bytes)) {
                copied = copy_folio_from_iter_atomic(folio, offset, bytes, i);

So we do still cond_resched(), but we might go up to PMD_SIZE
between calls.  This is new code in 6.6 so it hasn't seen use by too
many users yet, but it's certainly bigger than the 16 pages used by
copy_chunked_from_user().  I honestly hadn't thought about preemption
latency.