:> 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.
: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.
-Matt
Matthew Dillon
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