Re: [PATCH 2/2] drm/panfrost: Add madvise and shrinker support

[Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>]

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> +/* madvise provides a way to tell the kernel in case a buffers contents
> + * can be discarded under memory pressure, which is useful for userspace
> + * bo cache where we want to optimistically hold on to buffer allocate
> + * and potential mmap, but allow the pages to be discarded under memory
> + * pressure.
> + *
> + * Typical usage would involve madvise(DONTNEED) when buffer enters BO
> + * cache, and madvise(WILLNEED) if trying to recycle buffer from BO cache.
> + * In the WILLNEED case, 'retained' indicates to userspace whether the
> + * backing pages still exist.
> + */
> +#define PANFROST_MADV_WILLNEED 0	/* backing pages are needed, status returned in 'retained' */
> +#define PANFROST_MADV_DONTNEED 1	/* backing pages not needed */
> +
> +struct drm_panfrost_madvise {
> +	__u32 handle;         /* in, GEM handle */
> +	__u32 madv;           /* in, PANFROST_MADV_x */
> +	__u32 retained;       /* out, whether backing store still exists */
> +};

Just to clarify about the `retained` flag: if userspace does a
madvise(WILLNEED) and we find out that retained=0, what's supposed to
happen?

Should userspace evict the BO from its local cache and allocate one
fresh? Or just remmap? Or something else?

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