Consistency of loops in mm/truncate.c?

[Hugh Dickins]

Andrew,

I have a series aimed at 2.6.41 to remove mm/shmem.c's peculiar radix
tree of swap entries, using slots in the file's standard radix_tree
instead - prompted in part by https://lkml.org/lkml/2011/1/22/110

There's a patch to give shmem its own truncation loop, handling pages
and swap entries in the same pass.  For that I want to start from a
copy of truncate_inode_page_range(), but notice some discrepancies
between the different loops in mm/truncate.c, so want to standardize
them first before copying.

The advancement of index is hard to follow: we rely upon page->index
of an unlocked page persisting, yet we're ashamed of doing so, sometimes
reading it again once locked.  invalidate_mapping_pages() apologizes for
this, but I think we should now just document that page->index is not
modified until the page is freed.

invalidate_inode_pages2_range() has two sophistications not seen
elsewhere, which 7afadfdc says were folded in by akpm (along with
a page->index one):

- Don't look up more pages than we're going to use:
  seems a good thing for me to fold into truncate_inode_pages_range()
  and invalidate_mapping_pages() too.

- Check for the cursor wrapping at the end of the mapping:
  but with

#if BITS_PER_LONG==32
#define MAX_LFS_FILESIZE (((u64)PAGE_CACHE_SIZE << (BITS_PER_LONG-1))-1) 
#elif BITS_PER_LONG==64
#define MAX_LFS_FILESIZE 0x7fffffffffffffffUL
#endif

  I don't see how page->index + 1 would ever be 0, even if one or
  other of those "-1"s went away; so may I delete the "wrapped" case?

Thanks,
Hugh
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