On Mon, Jul 15, 2024 at 04:59:36PM +0100, Matthew Wilcox wrote: > I'm looking at ->write_begin() / ->write_end() again. Here are our > current callers: > > drivers/gpu/drm/i915/gem/i915_gem_shmem.c: > [1] shmem_pwrite() > [2] i915_gem_object_create_shmem_from_data() These really need to use actual shmem exported APIs, probably shmem_get_folio, instead of abusing the aops. With that we can then easily kill ->write_begin() / ->write_end() for shmem. > fs/affs/file.c: Most of these fs-specific ones should really hardcode the calls to the usually once or sometimes few potential instances that could be called so that we can devirtualize the alls. > fs/buffer.c: > [4] generic_cont_expand_simple() > [5] cont_expand_zero() > [6] cont_expand_zero() > fs/namei.c: > [B] page_symlink() > The copy_from_user() / memcpy() users feel like they should all end > up calling ->write_iter(). > One way they could do this is by calling > kernel_write() / __kernel_write(), but I'm not sure whether they > should have the various accounting things (add_wchar(), inc_syscw()) > that happen inside __kernel_write_iter(). > They often sit much lower in the stack and/or are used for files that don't have a ->write_iter. e.g. page_symlink is obviously used for symlinks that don't have ->write_iter. For generic_cont_expand_simple goins through write_iter might be an option, but instead of going through file ops the better idea might be to just pass a write_iter-prototyped callback directly to it. cont_expand_zero is a helper for cont_write_begin, which is used to implement ->write_begin, so this actually already is a recursion, adding another indirect to it is probably not helpful.
