On Tue, Dec 05, 2017 at 07:34:01AM -0800, Matthew Wilcox wrote: > > I don't see a discussion of this anywhere ... and I thought linux-api was > the right place to ask. > > I see holepunch as being "like ftruncate() but for the middle of the file". > So it feels weird to me that MAP_PRIVATE behaves differently; truncate > will rip out COWed pages, but holepunch won't. Is there a good reason for > this behaviour, and is it too late to change? I see truncate as 'change this global size property of a file' which is why we'd rip out COWed pages past the new EOF, whereas punch hole is more akin to writing zeroes to a certain file range. This shouldn't affect private mappings that a process has already written to and therefore gotten its own copy... ...but the manpage says "It is unspecified whether changes made to the file after the mmap() call are visible in the mapped region." so I guess either way is fine? Though who knows what now depends on the current behavior, so changing it would be hard. Thinking about this more, it seems weird that if you punch a range starting before the cow'd page and ending after the cow'd page that a reread of that part of the shared mapping now gets you zeroed pages except for the cow'd page which is still intact...? <shrug> --D > -- > To unsubscribe from this list: send the line "unsubscribe linux-api" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
