Messages in this thread | | | Date | Wed, 18 Jun 2008 17:20:41 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: Cyrus mmap vs lseek/write usage - (WAS: BUG: mmapfile/writev spurious zero bytes (x86_64/not i386, bisected, reproducable)) |
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On Thu, 19 Jun 2008, Robert Mueller wrote: > > As noted above, one thing cyrus does which does seem to be plain "wrong" > is that it mmaps a region greater the file size (rounds to an 8k > boundary, but 8k-16k past the current end of the file) and then assumes > that when it writes to the end of the file (but less than the end of the > mmap region) that there's no need to remmap and that data is immediately > available within the previous mmaped region.
Pretty much any OS that tries to be make mmap() coherent with regular read/write accesses will automatically also have to be coherent wrt file size updates.
IOW, I don't think that cyrus is real any more "wrong" in this than in assuming that you can mix read/write and mmap() accesses. In fact, I suspect that Cyrus is probably _more_ conservative than most, in that it would not be totally unheard of to just do a much bigger mmap(), and not even bother to re-do it until the file grows past that size (ie no 8k/16k granularity, but make it arbitrarily non-granular).
> Apparently that works on most OS's (but is what this bug actually > exposed), but according to the mmap docs: > > --- > If the size of the mapped file changes after the call to mmap() as a > result of some other operation on the mapped file, the effect of > references to portions of the mapped region that correspond to added or > removed portions of the file is unspecified.
Note that if you really want to be portable, you simply must not mix mmap() with *any* other operations without sprinking in a healthy amount of "msync()" or unmapping/remapping entirely.
So _in_practice_ - because everybody tries to do a good job - you can actually expect to have mmap() be coherent, even though there are no real guarantees.
> Amazingly (apart from HP/UX) no OS actually seems to have a problem with > this since there would be massive cyrus bug reports otherwise.
Yeah. Over the years, the pain from having a non-coherent mmap() generally has pushed everybody into just making mmap() easy to use. Which means that mixing things generally works fine, even if it is not at all _guaranteed_.
So I'd expect mmap+write to work and be coherent almost always. But it's still a fairly unusual combination, and I would personally think that using MAP_SHARED and writing through the mmap() would be the less surprising model.
Linus
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