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    SubjectRe: [PATCH -mm 1/3] swsusp: Add ioctl for swap files support
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    On Friday, 29 September 2006 00:42, Andrew Morton wrote:
    > On Fri, 29 Sep 2006 00:13:38 +0200
    > "Rafael J. Wysocki" <rjw@sisk.pl> wrote:
    >
    > > To be able to use swap files as suspend storage from the userland suspend
    > > tools we need an additional ioctl() that will allow us to provide the kernel
    > > with both the swap header's offset and the identification of the resume
    > > partition.
    > >
    > > The new ioctl() should be regarded as a replacement for the
    > > SNAPSHOT_SET_SWAP_FILE ioctl() that from now on will be considered as
    > > obsolete, but has to stay for backwards compatibility of the interface.
    > >
    > > +
    > > +/*
    > > + * This structure is used to pass the values needed for the identification
    > > + * of the resume swap area from a user space to the kernel via the
    > > + * SNAPSHOT_SET_SWAP_AREA ioctl
    > > + */
    > > +struct resume_swap_area {
    > > + u_int16_t dev;
    > > + loff_t offset;
    > > +} __attribute__((packed));
    > >
    >
    > hmm. Asking the compiler to pack 16-bit and 64-bit quantities in this
    > manner is a bit risky. I guess it'll do the right thing, consistently,
    > across all compiler versions and vendors and 32-bit-on-64-bit-kernel, etc.
    >
    > But from a defensiveness/paranoia POV it'd be better to use a u32 here, I
    > suspect. (Will access to that loff_t cause an alignment trap on ia64? Any
    > other CPUs? Dunno).

    I'm not sure too. Will change it to a 32-bit value.

    >
    > > #define PMOPS_PREPARE 1
    > > #define PMOPS_ENTER 2
    > > Index: linux-2.6.18-mm1/kernel/power/user.c
    > > ===================================================================
    > > --- linux-2.6.18-mm1.orig/kernel/power/user.c
    > > +++ linux-2.6.18-mm1/kernel/power/user.c
    > > @@ -343,6 +343,37 @@ OutS3:
    > > }
    > > break;
    > >
    > > + case SNAPSHOT_SET_SWAP_AREA:
    > > + if (data->bitmap) {
    > > + error = -EPERM;
    > > + } else {
    > > + struct resume_swap_area swap_area;
    > > + dev_t swdev;
    > > +
    > > + error = copy_from_user(&swap_area, (void __user *)arg,
    > > + sizeof(struct resume_swap_area));
    > > + if (error) {
    > > + error = -EFAULT;
    > > + break;
    > > + }
    > > +
    > > + /*
    > > + * User space encodes device types as two-byte values,
    > > + * so we need to recode them
    > > + */
    >
    > Really? stat() uses unsigned long and stat64() uses unsigned long long dev_t.

    I don't know what the remaining 6 bytes are used for. Anyway we need to use
    old_decode_dev() to get this right.

    > > + swdev = old_decode_dev(swap_area.dev);
    > > + if (swdev) {
    > > + offset = swap_area.offset;
    > > + data->swap = swap_type_of(swdev, offset);
    > > + if (data->swap < 0)
    > > + error = -ENODEV;
    > > + } else {
    > > + data->swap = -1;
    > > + error = -EINVAL;
    > > + }
    > > + }
    > > + break;
    > > +
    > > default:
    > > error = -ENOTTY;
    >
    > But I wonder if we need to pass the device identified into this ioctl at
    > all. What device is the ioctl() against? ie: what do `filp' and `inode'
    > point at? If it's /dev/hda1 then everything we need is right there, is it
    > not?
    >
    > ohshit, it's a miscdevice. I wonder if it would have defined all this
    > stuff to be operations against the blockdev. Perhaps not.

    Nope. We need char-like read() and write().

    > Well anyway. It might be neater to require that userspace open /dev/hda1
    > and pass in the fd to this ioctl. But this code will never be neat, so
    > whatever.

    :-)
    -
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