Re: [PATCH] mm/filemap: clarify filemap_fault() comments for not uptodate case

[Matthew Wilcox <willy@infradead.org>]

On Sun, Oct 01, 2023 at 12:10:29AM +0100, Lorenzo Stoakes wrote:
> The existing comments in filemap_fault() suggest that, after either a minor
> fault has occurred and filemap_get_folio() found a folio in the page cache,
> or a major fault arose and __filemap_get_folio(FGP_CREATE...) did the job
> (having relied on do_sync_mmap_readahead() or filemap_read_folio() to read
> in the folio), the only possible reason it could not be uptodate is because
> of an error.
> 
> This is not so, as if, for instance, the fault occurred within a VMA which
> had the VM_RAND_READ flag set (via madvise() with the MADV_RANDOM flag
> specified), this would cause even synchronous readahead to fail to read in
> the folio.
> 
> I confirmed this by dropping page caches and faulting in memory madvise()'d
> this way, observing that this code path was reached on each occasion.
> 
> Clarify the comments to include this case, and additionally update the
> comment recently added around the invalidate lock logic to make it clear
> the comment explicitly refers to the minor fault case.

I do appreciate the comment being made accurate, but I wonder if we
shouldn't change the code to match the comment.  We're got two "should"s
pointing in different directions -- we "should" not use readahead if
readahead is disabled, and we "should" always use readahead first,
using read_folio() only if readahead doesn't succeed.

The code isn't ideally structured for this, but I'm going to play with
it a bit and see what I can create.