io_uring/rsrc: don't rely on user vaddr alignment

There is no guaranteed alignment for user pointers, however the
calculation of an offset of the first page into a folio after coalescing
uses some weird bit mask logic, get rid of it.

Cc: [email protected]
Reported-by: David Hildenbrand <[email protected]>
Fixes: a8edbb4 ("io_uring/rsrc: enable multi-hugepage buffer coalescing")
Signed-off-by: Pavel Begunkov <[email protected]>
Link: https://lore.kernel.org/io-uring/e387b4c78b33f231105a601d84eefd8301f57954.1750771718.git.asml.silence@gmail.com/
Signed-off-by: Jens Axboe <[email protected]>

Author: isilence

io_uring/rsrc.c

@@ -734,6 +734,7 @@ bool io_check_coalesce_buffer(struct page **page_array, int nr_pages,
 
 	data->nr_pages_mid = folio_nr_pages(folio);
 	data->folio_shift = folio_shift(folio);
+	data->first_folio_page_idx = folio_page_idx(folio, page_array[0]);
 
 	/*
 	 * Check if pages are contiguous inside a folio, and all folios have
@@ -827,7 +828,11 @@ static struct io_rsrc_node *io_sqe_buffer_register(struct io_ring_ctx *ctx,
 	if (coalesced)
 		imu->folio_shift = data.folio_shift;
 	refcount_set(&imu->refs, 1);
-	off = (unsigned long) iov->iov_base & ((1UL << imu->folio_shift) - 1);
+
+	off = (unsigned long)iov->iov_base & ~PAGE_MASK;
+	if (coalesced)
+		off += data.first_folio_page_idx << PAGE_SHIFT;
+
 	node->buf = imu;
 	ret = 0;
 

io_uring/rsrc.h

@@ -49,6 +49,7 @@ struct io_imu_folio_data {
 	unsigned int	nr_pages_mid;
 	unsigned int	folio_shift;
 	unsigned int	nr_folios;
+	unsigned long	first_folio_page_idx;
 };
 
 bool io_rsrc_cache_init(struct io_ring_ctx *ctx);