On Tue, Feb 14, 2023 at 06:21:50PM +0530, Charan Teja Kalla wrote: > Currently fadvise(2) is supported only for the files that doesn't > associated with noop_backing_dev_info thus for the files, like shmem, > fadvise results into NOP. But then there is file_operations->fadvise() > that lets the file systems to implement their own fadvise > implementation. Use this support to implement some of the POSIX_FADV_XXX > functionality for shmem files. > > This patch aims to implement POSIX_FADV_WILLNEED and POSIX_FADV_DONTNEED > advices to shmem files which can be helpful for the clients who may want > to manage the shmem pages of the files that are created through > shmem_file_setup[_with_mnt](). One usecase is implemented on the > Snapdragon SoC's running Android where the graphics client is allocating > lot of shmem pages per process and pinning them. When this process is > put to background, the instantaneous reclaim is performed on those shmem > pages using the logic implemented downstream[3][4]. With this patch, the > client can now issue the fadvise calls on the shmem files that does the > instantaneous reclaim which can aid the use cases like mentioned above. > > This usecase lead to ~2% reduction in average launch latencies of the > apps and 10% in total number of kills by the low memory killer running > on Android. > > Some questions asked while reviewing this patch: > Q) Can the same thing be achieved with FD mapped to user and use > madvise? > A) All drivers are not mapping all the shmem fd's to user space and want > to manage them with in the kernel. Ex: shmem memory can be mapped to the > other subsystems and they fill in the data and then give it to other > subsystem for further processing, where, the user mapping is not at all > required. A simple example, memory that is given for gpu subsystem > which can be filled directly and give to display subsystem. And the > respective drivers know well about when to keep that memory in ram or > swap based on may be a user activity. > > Q) Should we add the documentation section in Manual pages? > A) The man[1] pages for the fadvise() whatever says is also applicable > for shmem files. so couldn't feel it correct to add specific to shmem > files separately. > > Q) The proposed semantics of POSIX_FADV_DONTNEED is actually similar to > MADV_PAGEOUT and different from MADV_DONTNEED. This is a user facing API > and this difference will cause confusion? > A) man pages [2] says that "POSIX_FADV_DONTNEED attempts to free cached > pages associated with the specified region." This means on issuing this > FADV, it is expected to free the file cache pages. And it is > implementation defined If the dirty pages may be attempted to writeback. > And the unwritten dirty pages will not be freed. So, FADV_DONTNEED also > covers the semantics of MADV_PAGEOUT for file pages and there is no > purpose of PAGEOUT for file pages. > > [1] https://linux.die.net/man/2/fadvise > [2] https://man7.org/linux/man-pages/man2/posix_fadvise.2.html > [3] https://git.codelinaro.org/clo/la/platform/vendor/qcom/opensource/graphics-kernel/-/blob/gfx-kernel.lnx.1.0.r3-rel/kgsl_reclaim.c#L289 > [4] https://android.googlesource.com/kernel/common/+/refs/heads/android12-5.10/mm/shmem.c#4310 > > Signed-off-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx> I am not familar with why the shmem has noop_backing_dev_info but the below code to reclaim shmem pages and POXIS_FADV_DONTNEED semantic looks correct for me. Only nit is the description covers mostly DONTNEED case but not WILLNEED case.
