Hi, This is never going to happen as per my understanding. You need to know internals of write system call for that. Lets say you have a huge buffer to write and you triggered write system call. Internally generic_file_write is called the page is allocated in kernel space. Your buffer is cut down into the PAGE_SIZE buffers for example a buffer of 40k and PAGE_SIZE is 4k then 10 pages are allocated. The page allocation is followed by prepare_write and commit_write. Now If your system is under pressure which may occur at page allocation. Then all the dirty inode are flushed to the storage. For that writepage is called on each inodes address space.This way write can request any huge buffer.Yes you can avoid to go through Page cache by O_DIRECT flag. But i am not clear about that mechanism. Prasanna --- stackframe@xxxxxxxxxxx wrote: > How is it that a write (man 2 write) can be passed a > buffer > that exceeds most of the physical and virtual memory > on a > system and work successfully? > > I wrote a simple program that allocates a large > buffer on > the heap, zeros the space and then calls write > (man 2 write). Even though the buffer is almost as > large > as physical memory + page space the call completes > without > error. In my example I allocated 705MB on a system > that > has 256MB physical and 512MB virtual. When it > completes > there is a 705MB file on disk as expected. > > So is it in the case of the sys_write that the > memory is > accessed and allowed to page fault as needed while > running > in the context of the kernel? Most of the kernel > books > talk about using copy_from_user on system calls > before > using memory from the user. Strace shows one write > call with the entire length. > > So what is going on to allow this to happen? > > Thanks, > Tony > > > -- > Kernelnewbies: Help each other learn about the Linux > kernel. > Archive: > http://mail.nl.linux.org/kernelnewbies/ > FAQ: http://kernelnewbies.org/faq/ > > __________________________________ Do you Yahoo!? Yahoo! Small Business - Try our new resources site! http://smallbusiness.yahoo.com/resources/ -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
