Re: mmap call to filesystem
Re: mmap call to filesystem
- Subject: Re: mmap call to filesystem
- From: "shailesh jain" <email@hidden>
- Date: Sat, 28 Jun 2008 19:33:39 -0700
On Sat, Jun 28, 2008 at 7:09 PM, Michael Smith <
email@hidden> wrote:
On Jun 28, 2008, at 5:09 PM, shailesh jain wrote:
2) Also, I wanted to know about the option MAP_PRIVATE passed to the mmap call. When, this option
is passed, the modifications to the data are just made in-memory and are not reflected to the actual object. Now, If PROT_WRITE option is passed to the filesystem and the file is just opened for reading,
should the call fail ?
For mmap to succeed, the file should have been opened with the read flag always.
Shouldn't it depend upon options {MAP_PRIVATE or MAP_SHARED } which are not passed to the filesystem ?
No. The VM is responsible for deciding whether pages will be written to the file; the filesystem is generally not involved in authorisation.
Also, I was looking at the mmap implementation but couldn't figure out how MAP_PRIVATE option was handled by kernel.
You're looking at the wrong place; you want to look at how the VM sets up the mapping against the object. You can expect to see the region marked copy-on-write, with an intermediate copy object involved.
= Mike
So, if the filesystem is not involved in authorization that what is the purpose of passing protection flag {PROT_WRITE, PROT_READ, PROT_EXEC} to the filesystem ?
/Shail
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