On Mon, 28 Jun 1999, Matthew Dillon wrote:
> :> it is extremely memory efficient.
> :
> :I guess you are talking about VMIO buffers where the pages are found and
> :registered into the buffer header during allocbuf(). When we do I/O on
> :VMIO buffers using conventional system call method, we specify UIO_NOCOPY
> :to instruct the uiomove() do not perform data copy.
>
> UIO_NOCOPY is used to handle a degenerate case in the VFS/BIO vs VM
> interaction for I/O, it has nothing to do with the read() or write()
> syscall per say, nor is it related to the mmap code.
>
> :> Programmers who use mmap() expect it to be as close to optimal as
> :> possible.
> :
> :I write a program to test the mmap() today. It turns out that a user can
> :modify the part of the mmapped area that is within the system returned
> :area but not part of the user-specified area.
> :
> :As I understand it, there are two access paths to a file: conventional I/O
> :through read/write systems calls and memory-mapped I/O. Both of them
> :converge at the vnode read and write routine (VOP_READ() and VOP_WRITE()).
> :This should give us the opportunity to guard against illegal memory-mapped
> :I/O writes made by the user.
>
> They converge in the VMIO page cache.
By converge, I mean VOP_GETPAGES() and VOP_PUTPAGES() will call VOP_READ()
and VOP_WRITE() just as read() and write() system call.
>
> :Maybe we can add some fields in the vm_object to record the real or
> :user-specifed area which can be passed to the vnode read and write
> :routine. In the vnode I/O routine, we should be able to limit the write to
> :only the orginal part of the area specified by the user. This practice
> :should not incur any performance loss.
> :
> :-Zhihui
>
> mmap bypasses the vnode. What you propose will not work because even if
> the VM object is process-specific, the pages underlying the VM object are
> not. If several processes are mmap()ing overlapping portions of the file,
> they are *sharing* the pages. So even though they are not sharing the
> VM object, the VM system will not be able to tell which process modified
> the page, and therefore any byte-ranged limits specified in the VM object
> will be useless.
This is a good point! I have never thought of it before. Thanks.
-Zhihui
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