On Tue, Oct 15, 2024 at 06:41:06PM +0200, Pankaj Raghav (Samsung) wrote: v14? Where are v1..13 of this patch? It's the first time I've seen it. > The readahead flag is set on a folio based on the lookahead_size and > nr_to_read. For example, when the readahead happens from index to index > + nr_to_read, then the readahead `mark` offset from index is set at > nr_to_read - lookahead_size. > > There are some scenarios where the lookahead_size > nr_to_read. If this > happens, readahead flag is not set on any folio on the current > readahead window. Please describe those scenarios, as that's the important bit. > There are two problems at the moment in the way `mark` is calculated > when lookahead_size > nr_to_read: > > - unsigned long `mark` will be assigned a negative value which can lead > to unexpected results in extreme cases due to wrap around. Can such an extreme case happen? > - The current calculation for `mark` with mapping_min_order > 0 gives > incorrect results when lookahead_size > nr_to_read due to rounding > up operation. > > Explicitly initialize `mark` to be ULONG_MAX and only calculate it > when lookahead_size is within the readahead window. You haven't really spelled out the consequences of this properly. Perhaps a worked example would help. I think the worst case scenario is that we set the flag on the wrong folio, causing readahead to occur when it should not. But maybe I'm wrong?
