On 6/3/25 16:28, Christoph Hellwig wrote: > On Tue, Jun 03, 2025 at 04:18:22PM +0200, Christian König wrote: >>> Does it matter compared to the I/O in this case? >> >> It unfortunately does, see the numbers on patch 3 and 4. > > That's kinda weird. Why does the page table lookup tage so much > time compared to normal I/O?
I have absolutely no idea. It's rather surprising for me as well. The user seems to have a rather slow CPU paired with fast I/O, but it still looks rather fishy to me. Additional to that allocating memory through memfd_create() is *much* slower on that box than through dma-buf-heaps (which basically just uses GFP and an array). We have seen something similar with customers systems which we couldn't explain so far. >> My question is rather if it's ok to call f_op->write_iter() and >> f_op->read_iter() with pages allocated by alloc_pages(), e.g. >> where drivers potentially ignore the page count and just re-use pages >> as they like? > > read_iter and write_iter with ITER_BVEC just use the pages as source > and destination of the I/O. They must not touch the refcounts or > do anything fancy with them. Various places in the kernel rely on > that. Perfect, thanks for that info. Regards, Christian.
