Re: Cyrus mmap vs lseek/write usage - (WAS: BUG: mmapfile/writevspurious zero bytes (x86_64/not i386, bisected, reproducable))

[Linus Torvalds]

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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