On Sun, May 23, 2010 at 9:09 AM, Jens Axboe <jens.axboe@xxxxxxxxxx> wrote: > On Sun, May 23 2010, Michael Kerrisk wrote: >> On Sun, May 23, 2010 at 4:38 AM, Andrew Morton >> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: >> > On Sun, 23 May 2010 07:30:01 +0200 Michael Kerrisk <mtk.manpages@xxxxxxxxx> wrote: >> > >> >> Hi all, >> >> >> >> I see that this patch has hit Linus's git, so some questions >> >> >> >> On Wed, May 19, 2010 at 6:49 PM, Linus Torvalds >> >> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >> >> > >> >> > >> >> > On Wed, 19 May 2010, Miklos Szeredi wrote: >> >> >> >> >> >> One issue I see is that it's possible to grow pipes indefinitely. >> >> >> Should this be restricted to privileged users? >> >> > >> >> > Yes. But perhaps only if it grows past the default (or perhaps "default*2" >> >> > or similar). That way a normal user could shrink the pipe buffers, and >> >> > then grow them again if he wants to. >> >> > >> >> > Oh, and I think you need to also require that there be at least two >> >> > buffers. Otherwise we can't guarantee POSIX behavior, I think. >> >> >> >> Is there any documentation (e.g., a man-pages patch) for these changes? >> >> >> >> The argument of the fcntl() operations is expressed in pages. I take >> >> it that this means that the semantics of the argument will very >> >> depending on the system page size? So for example, 2 on x86 will mean >> >> 8192 bytes, but will mean 32768 of ia64? That seems very weird. (And >> >> what about architectures where the page size is switchable?) Such >> >> changes in semantics should not be silent for the use, IMO. >> > >> > Well, there is getpagesize(). But I agree - this interface is just >> > asking (x86) people to write non-portable code. >> > >> > otoh, if the arg was in bytes, they'd just hard-code "8192". They're >> > clever like that. >> > >> > But we have gone to some lengths to avoid exposing things like >> > PAGE_SIZE and HZ in procfs, so it makes sense to take the same approach >> > to syscalls. >> >> Quite. All of the other memory-related APIs that I can think of >> require the user to express the info in bytes. (mlock(), >> remap_file_pages(), mmap(), mremap(), mprotect(), shmget(), and so >> on). Not doing the same for this interface is needlessly inconsistent. >> And while there will be the silly users you mention above, smart users >> will know how to do the right thing with a consistently designed >> interface. > > We can easily make F_GETPIPE_SZ return bytes, but I don't think passing > in bytes to F_SETPIPE_SZ makes a lot of sense. The pipe array must be a > power of 2 in pages. So the question is if that makes the API cleaner, > passing in number of pages but returning bytes? Or pass in bytes all > around, but have F_SETPIPE_SZ round to the nearest multiple of pow2 in > pages if need be. Then it would return a size at least what was passed > in, or error. I'd recommend this: Pass it in and out in bytes. Don't round to a power of 2. Require the user to know what they are doing. Give an error if the user doesn't supply a power-of-2 * page-size for F_SETPIPE_SZ. (Again, consider the case of architectures with switchable page sizes.) Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Author of "The Linux Programming Interface" http://blog.man7.org/ -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
