Revert "readahead: properly shorten readahead when falling back to do_page_cache_ra()"

JIRA: https://issues.redhat.com/browse/RHEL-55724
JIRA: https://issues.redhat.com/browse/RHEL-56929
CVE: CVE-2024-57839

commit a220d6b
Author: Jan Kara <jack@suse.cz>
Date:   Tue Nov 26 15:52:08 2024 +0100

    Revert "readahead: properly shorten readahead when falling back to do_page_cache_ra()"

    This reverts commit 7c87758.

    Anders and Philippe have reported that recent kernels occasionally hang
    when used with NFS in readahead code.  The problem has been bisected to
    7c87758 ("readahead: properly shorten readahead when falling back to
    do_page_cache_ra()").  The cause of the problem is that ra->size can be
    shrunk by read_pages() call and subsequently we end up calling
    do_page_cache_ra() with negative (read huge positive) number of pages.
    Let's revert 7c87758 for now until we can find a proper way how the
    logic in read_pages() and page_cache_ra_order() can coexist.  This can
    lead to reduced readahead throughput due to readahead window confusion but
    that's better than outright hangs.

    Link: https://lkml.kernel.org/r/20241126145208.985-1-jack@suse.cz
    Fixes: 7c87758 ("readahead: properly shorten readahead when falling back to do_page_cache_ra()")
    Reported-by: Anders Blomdell <anders.blomdell@gmail.com>
    Reported-by: Philippe Troin <phil@fifi.org>
    Signed-off-by: Jan Kara <jack@suse.cz>
    Tested-by: Philippe Troin <phil@fifi.org>
    Cc: Matthew Wilcox <willy@infradead.org>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

Author: Maxim Levitsky

mm/readahead.c

@@ -453,8 +453,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
 		struct file_ra_state *ra, unsigned int new_order)
 {
 	struct address_space *mapping = ractl->mapping;
-	pgoff_t start = readahead_index(ractl);
-	pgoff_t index = start;
+	pgoff_t index = readahead_index(ractl);
 	unsigned int min_order = mapping_min_folio_order(mapping);
 	pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
 	pgoff_t mark = index + ra->size - ra->async_size;
@@ -517,7 +516,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
 	if (!err)
 		return;
 fallback:
-	do_page_cache_ra(ractl, ra->size - (index - start), ra->async_size);
+	do_page_cache_ra(ractl, ra->size, ra->async_size);
 }
 
 static unsigned long ractl_max_pages(struct readahead_control *ractl,