Date: Tue, 15 Jul 2008 12:34:02 +0300 From: Kostik Belousov <kostikbel@gmail.com> To: Stacey Son <sson@freebsd.org> Cc: Andrew Gallatin <gallatin@cs.duke.edu>, freebsd-arch@freebsd.org Subject: Re: ksyms pseudo driver Message-ID: <20080715093402.GO17123@deviant.kiev.zoral.com.ua> In-Reply-To: <487AD49F.6040304@freebsd.org> References: <4875A5D2.8030902@freebsd.org> <20080711155232.A96384@grasshopper.cs.duke.edu> <48780661.5050002@freebsd.org> <20080712045837.GD17123@deviant.kiev.zoral.com.ua> <487AD49F.6040304@freebsd.org>
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--tlBB11+jzEStP2nB Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Sun, Jul 13, 2008 at 11:22:55PM -0500, Stacey Son wrote: > Kostik Belousov wrote: > >Most likely, I miss some obvious reason there. But for me it looks > >like you do it in the reverse. The natural setup would be to require > >userspace to supply an allocated memory to the driver, and then the > >driver fills the memory with symbol table. This solves the problem of > >exhaustion of kernel address space. > > =20 >=20 > The snapshot of the consolidated symbol table is made when /dev/ksyms is= =20 > opened. The storage for the snapshot is allocated in the memory map of= =20 > the calling process. No kernel address space is used for the snapshot. Again, why this is done this way ? Why not creating snapshot when the user process issues ioctl that supplies neccessary usermode memory to the driver ? >=20 > A temporary buffer is allocated in kernel space in the read() handler=20 > (ksyms_read). Right now, for a read, it does two copies: one from=20 > user space to the temporary kernel space buffer and a second copy from=20 > the kernel space temp buffer and back out to user space. Ideally, it=20 > would be nice to do just one user space to user space copy directly in=20 > the kernel. >=20 > >As usual, when user-supplied region is too small, driver shall return > >both an error and new required size. It is understandable that the size > >is volatile and may be too small for the next call too. But, in fact, > >kernel symtable does not change too often, so I think even the one > >iteration mostly succeed. > > =20 >=20 > The reason the driver tries three times to create a valid snapshot is I= =20 > couldn't figure out a way (without creating a lock reversal) to=20 > temporarily keep modules from being loaded or unloaded while the=20 > snapshot is created. I agree that it should be able to create the=20 > snapshot on the first iteration in most cases. >=20 > BTW, you may have noticed the ksyms driver now uses your per-open file=20 > private data code which I like much better than using clone_create() for= =20 > per-descriptor storage. Does it work ? Do you have any suggestions for the KPI ? --tlBB11+jzEStP2nB Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (FreeBSD) iEYEARECAAYFAkh8bwkACgkQC3+MBN1Mb4jKCACgk8JxGJf2CHd/JB31ouYKxw5J 7ikAoJtSodf1j2gW1I3xUqNRwA2UMLqO =2azh -----END PGP SIGNATURE----- --tlBB11+jzEStP2nB--
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