On Fri, May 22, 2020 at 08:49:06AM +1000, Dave Chinner wrote: > Ok, so the main issue I have with the filesystem/iomap side of > things is that it appears to be adding "transparent huge page" > awareness to the filesysetm code, not "large page support". > > For people that aren't aware of the difference between the > transparent huge and and a normal compound page (e.g. I have no idea > what the difference is), this is likely to cause problems, > especially as you haven't explained at all in this description why > transparent huge pages are being used rather than bog standard > compound pages. The primary reason to use a different name from compound_* is so that it can be compiled out for systems that don't enable CONFIG_TRANSPARENT_HUGEPAGE. So THPs are compound pages, as they always have been, but for a filesystem, using thp_size() will compile to either page_size() or PAGE_SIZE depending on CONFIG_TRANSPARENT_HUGEPAGE. Now, maybe thp_size() is the wrong name, but then you need to suggest a better name ;-) > And, really, why should iomap or the filesystems care if the large > page is a THP or just a high order compound page? The interface > for operating on these things at the page cache level should be the > same. We already have page_size() and friends for operating on > high order compound pages, yet the iomap stuff has this new > thp_size() function instead of just using page_size(). THis is going > to lead to confusion and future bugs when people who don't know the > difference use the wrong page size function in their filesystem > code. There is no wrong function to use -- just one that expands to more code in the case where CONFIG_TRANSPARENT_HUGEPAGE is disabled.
