Re: [PATCH] exec: fetch the alignment of Linux devdax pmem character device nodes

[Liu, Jingqi]

On 4/7/2020 4:08 PM, Paolo Bonzini wrote:
> On 07/04/20 09:29, Liu, Jingqi wrote:
> > Ping.
> >
> > Any comments are appreciated.
> >
> > Hi Paolo, Richard,
> >
> > Any comments about this ?
> I was hoping to get a review from someone else because I have no way to
> test it.  But I've now queued the patch, thanks.

Hi Paolo,

Thanks for your comments.

Hi Dan,

Can you help review this patch ?

Thanks,

Jingqi

> Paolo
>
> > Thanks,
> >
> > Jingqi
> >
> > On 4/1/2020 11:13 AM, Liu, Jingqi wrote:
> > > If the backend file is devdax pmem character device, the alignment
> > > specified by the option 'align=NUM' in the '-object memory-backend-file'
> > > needs to match the alignment requirement of the devdax pmem character
> > > device.
> > >
> > > This patch fetches the devdax pmem file 'align', so that we can compare
> > > it with the NUM of 'align=NUM'.
> > > The NUM needs to be larger than or equal to the devdax pmem file 'align'.
> > >
> > > It also fixes the problem that mmap() returns failure in qemu_ram_mmap()
> > > when the NUM of 'align=NUM' is less than the devdax pmem file 'align'.
> > >
> > > Cc: Dan Williams <address@hidden>
> > > Signed-off-by: Jingqi Liu <address@hidden>
> > > ---
> > >    exec.c | 46 +++++++++++++++++++++++++++++++++++++++++++++-
> > >    1 file changed, 45 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/exec.c b/exec.c
> > > index de9d949902..8221abffec 100644
> > > --- a/exec.c
> > > +++ b/exec.c
> > > @@ -1736,6 +1736,42 @@ static int64_t get_file_size(int fd)
> > >        return size;
> > >    }
> > >    +static int64_t get_file_align(int fd)
> > > +{
> > > +    int64_t align = -1;
> > > +#if defined(__linux__)
> > > +    struct stat st;
> > > +
> > > +    if (fstat(fd, &st) < 0) {
> > > +        return -errno;
> > > +    }
> > > +
> > > +    /* Special handling for devdax character devices */
> > > +    if (S_ISCHR(st.st_mode)) {
> > > +        g_autofree char *subsystem_path = NULL;
> > > +        g_autofree char *subsystem = NULL;
> > > +
> > > +        subsystem_path =
> > > g_strdup_printf("/sys/dev/char/%d:%d/subsystem",
> > > +                                         major(st.st_rdev),
> > > minor(st.st_rdev));
> > > +        subsystem = g_file_read_link(subsystem_path, NULL);
> > > +
> > > +        if (subsystem && g_str_has_suffix(subsystem, "/dax")) {
> > > +            g_autofree char *align_path = NULL;
> > > +            g_autofree char *align_str = NULL;
> > > +
> > > +            align_path =
> > > g_strdup_printf("/sys/dev/char/%d:%d/device/align",
> > > +                                    major(st.st_rdev),
> > > minor(st.st_rdev));
> > > +
> > > +            if (g_file_get_contents(align_path, &align_str, NULL,
> > > NULL)) {
> > > +                return g_ascii_strtoll(align_str, NULL, 0);
> > > +            }
> > > +        }
> > > +    }
> > > +#endif /* defined(__linux__) */
> > > +
> > > +    return align;
> > > +}
> > > +
> > >    static int file_ram_open(const char *path,
> > >                             const char *region_name,
> > >                             bool *created,
> > > @@ -2275,7 +2311,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t
> > > size, MemoryRegion *mr,
> > >    {
> > >        RAMBlock *new_block;
> > >        Error *local_err = NULL;
> > > -    int64_t file_size;
> > > +    int64_t file_size, file_align;
> > >          /* Just support these ram flags by now. */
> > >        assert((ram_flags & ~(RAM_SHARED | RAM_PMEM)) == 0);
> > > @@ -2311,6 +2347,14 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t
> > > size, MemoryRegion *mr,
> > >            return NULL;
> > >        }
> > >    +    file_align = get_file_align(fd);
> > > +    if (file_align > 0 && mr && file_align > mr->align) {
> > > +        error_setg(errp, "backing store align 0x%" PRIx64
> > > +                   " is larger than 'align' option 0x" RAM_ADDR_FMT,
> > > +                   file_align, mr->align);
> > > +        return NULL;
> > > +    }
> > > +
> > >        new_block = g_malloc0(sizeof(*new_block));
> > >        new_block->mr = mr;
> > >        new_block->used_length = size;