Re: Getting a read call before open
Re: Getting a read call before open
- Subject: Re: Getting a read call before open
- From: "shailesh jain" <email@hidden>
- Date: Thu, 26 Jun 2008 12:42:38 -0700
Hi,
The follow up question, I guess.
Now I have just implemented a prototype VOP_PAGEIN. But the parameters passed to this function {size, offset, vm_offset} are all set to
zero. I am clueless, as why is it set to zero ?
Ideally shouldn't offset be 4096?, because first page has already been loaded by execve.
Note: My filesystem does not support caching. Also, let me know if this question is more appropriate on filesystem-dev mailing list.
/Shail
On Wed, Jun 25, 2008 at 11:16 PM, shailesh jain <
email@hidden> wrote:
Hi,
Thanks. I actually figured out that I had not yet implemented VOP_PAGEIN.
Thus, execve used to only load 1st page (4096 bytes) and then later depended on page fault
to load remaining bytes. But since I didn't implement VOP_PAGEIN, the application just used to hang.
Thanks for the information.
/ShailOn Wed, Jun 25, 2008 at 11:03 PM, Terry Lambert <
email@hidden> wrote:
On Jun 25, 2008, at 6:54 PM, shailesh jain wrote:
When I try to run executable over my filesystem, it just hangs (i.e shell prompt never returns) when I tried to do implicit open in the read call to my filesystem.
Digging through the source code, I found that execve calls vn_rdwr() and subsequently, VOP_READ() call. This read is invoked to load PAGESIZE bytes (4096) which my filesystem delivers it properly. However, I do not get read call to load remaining bytes. I can't seem to decipher that.
/Shail
On Wed, Jun 25, 2008 at 4:12 PM, shailesh jain <email@hidden> wrote:
Is it legitimate for a filesystem to get a read call before open call ? Also, how should a filesystem
handle such behavior (implicit opens and close ?)
If you vend a vnode, be prepared to get any number of calls upon it. Once it has been vended, yo have agreed to puture calls on it until such time as it has been released back t you for recycling.
Certain filesystems dislike this (SMB, as an example, disallows renames for open files, and we don't support ETXTBUSY unless the FS client maintains "this has been exec'ed state" and returns it itself). I understand that this can bother people, but until the vnode has been invalidated, either by being given back to you as no longer being needed by who you gave it to, or being deadfs'ed (e.g. by a forced unmount from your side of things), it is the property of whoever you vended it to.
Another situation where it's possible for this to happen is open/mmap/close, where the memory mapping is kept active by a vnode reference by the paging system (maybe you just did not notice the open/close, and now you incorrectly think it's closed). Again, as far as the kernel is concerned, the vnode pointer *is* the file.
For the specific case of exec, yes, there is a vn_rdwr() after you vended the open vnode to the exec code via a lookup of a name. You will potentially get VOP_READ() calls if it's a FAT file or you have code signing, and you will definitely get vm mappings established for the address space of the new process, so what you are seeing is both reasonable and expected.
If you choose to rip the vnode out from under us, of course whatever program you are running will crash the first time it page faults in a clean page from the backing store you promised it when you gave out the vp.
Hope this clears things up for you.
- Terry
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