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    SubjectRe: [PATCH 1/1] Create debugfs file with virtio balloon usage information
    On Mon, Jun 27, 2022 at 07:19:09PM +0000, Alexander Atanasov wrote:
    > Allow the guest to know how much it is ballooned by the host.
    > It is useful when debugging out of memory conditions.
    >
    > When host gets back memory from the guest it is accounted
    > as used memory in the guest but the guest have no way to know
    > how much it is actually ballooned.
    >
    > No pretty printing and fixed as per coding style.
    >
    > Signed-off-by: Alexander Atanasov <alexander.atanasov@virtuozzo.com>
    > ---
    > drivers/virtio/virtio_balloon.c | 71 +++++++++++++++++++++++++++++++++
    > 1 file changed, 71 insertions(+)
    >
    > - Leave pretty print to userspace
    > - Fixed coding style
    >
    > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
    > index b9737da6c4dd..1bb6a6d0e37b 100644
    > --- a/drivers/virtio/virtio_balloon.c
    > +++ b/drivers/virtio/virtio_balloon.c
    > @@ -10,6 +10,7 @@
    > #include <linux/virtio_balloon.h>
    > #include <linux/swap.h>
    > #include <linux/workqueue.h>
    > +#include <linux/debugfs.h>
    > #include <linux/delay.h>
    > #include <linux/slab.h>
    > #include <linux/module.h>
    > @@ -731,6 +732,71 @@ static void report_free_page_func(struct work_struct *work)
    > }
    > }
    >
    > +/*
    > + * DEBUGFS Interface
    > + */
    > +#ifdef CONFIG_DEBUG_FS
    > +
    > +/**
    > + * virtio_balloon_debug_show - shows statistics of balloon operations.
    > + * @f: pointer to the &struct seq_file.
    > + * @offset: ignored.
    > + *
    > + * Provides the statistics that can be accessed in virtio-balloon in the debugfs.
    > + *
    > + * Return: zero on success or an error code.
    > + */
    > +static int virtio_balloon_debug_show(struct seq_file *f, void *offset)
    > +{
    > + struct virtio_balloon *b = f->private;
    > + u32 num_pages;
    > + struct sysinfo i;
    > +
    > + si_meminfo(&i);
    > +
    > + seq_printf(f, "%-22s: %llx\n", "capabilities", b->vdev->features);

    why do we need this in debugfs? Isn't this available in sysfs already?

    > + seq_printf(f, "%-22s: %d\n", "page_size", 4096);

    I suspect this function doesn't work properly when page size is not 4K.

    > +
    > + virtio_cread_le(b->vdev, struct virtio_balloon_config, actual,
    > + &num_pages);
    > + /* Memory returned to host or amount we can inflate if possible */
    > + seq_printf(f, "%-22s: %u\n", "ballooned_pages", num_pages);

    I don't really get the comment here.

    > +
    > + /* Total Memory for the guest from host */
    > + seq_printf(f, "%-22s: %lu\n", "total_pages", i.totalram);
    > +
    > + /* Current memory for the guest */
    > + seq_printf(f, "%-22s: %lu\n", "current_pages", i.totalram - num_pages);
    Are you sure these are in units of 4Kbyte pages?

    > + return 0;
    > +}
    > +
    > +DEFINE_SHOW_ATTRIBUTE(virtio_balloon_debug);
    > +
    > +static void virtio_balloon_debugfs_init(struct virtio_balloon *b)
    > +{
    > + debugfs_create_file("virtio-balloon", 0444, NULL, b,
    > + &virtio_balloon_debug_fops);
    > +}
    > +
    > +static void virtio_balloon_debugfs_exit(struct virtio_balloon *b)
    > +{
    > + debugfs_remove(debugfs_lookup("virtio-balloon", NULL));
    > +}
    > +
    > +#else
    > +
    > +static inline void virtio_balloon_debugfs_init(struct virtio_balloon *b)
    > +{
    > +}
    > +
    > +static inline void virtio_balloon_debugfs_exit(struct virtio_balloon *b)
    > +{
    > +}
    > +
    > +#endif /* CONFIG_DEBUG_FS */
    > +
    > #ifdef CONFIG_BALLOON_COMPACTION
    > /*
    > * virtballoon_migratepage - perform the balloon page migration on behalf of
    > @@ -1019,6 +1085,9 @@ static int virtballoon_probe(struct virtio_device *vdev)
    >
    > if (towards_target(vb))
    > virtballoon_changed(vdev);
    > +
    > + virtio_balloon_debugfs_init(vb);
    > +
    > return 0;
    >
    > out_unregister_oom:
    > @@ -1065,6 +1134,8 @@ static void virtballoon_remove(struct virtio_device *vdev)
    > {
    > struct virtio_balloon *vb = vdev->priv;
    >
    > + virtio_balloon_debugfs_exit(vb);
    > +
    > if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING))
    > page_reporting_unregister(&vb->pr_dev_info);
    > if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
    > --
    > 2.25.1

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